Modern pop music is clearly dominated by three principle instruments: (1) Human voices, (2) Guitar instruments, and (3) Keyboard instruments. Virtually all popular music is derived from these three instruments, especialy if we consider the electric bass to be a form of guitar. (I would consider this obvious, since the modern electric bass bears closer constructional similarity to a guitar than to the acoustic, upright bass that once most commonly played the bass role in popular ensembles 50 and more years ago.)
Comparing the increasingly homogeneous modern music to the diversity of popular material that existed prior to about the 1960s can be depressing business from the standpoint of anyone who enjoys hearing something a little different from time to time. But, as I am about to argue, this phenomenon is based in part on the gravitation of people toward these three principle instruments.
The Evolution Of Instrument Popularity
At one point in time, the world's most popular instrument was the saxophone, which is astounding if you consider the fact that it was originally developed as a bit of a novelty. It rose to popularity through its widespread use in military bands, which appreciated its range and versatility when applied to that purpose. Note well that the driving force behind the popularity of the saxophone was the prevailing form of popular music at the time. As music progressed, poor American musicians started applying the saxophone to "Dixieland" music, the precursor to jazz. And, of course, jazz music took saxophones to their highest popularity.
There are some important compositional differences between jazz and military band music, beyond the obvious. Band music, like orchestral music, is typically arranged in such a way that everyone in the ensemble - often a very large number of people - is playing one not at a time. This is why most "big band" instruments are either drums or instruments that play only one note at a time, like saxophones, flutes, trumpets, trombones, etc. Jazz, on the other hand, employs the folk music compositional approach of using "rhythm instruments" (in the case of jazz, these would be the bass, the guitar, and the piano) to "comp" the underlying harmonic structure of the composition while "lead instruments" (saxophones, trumpets, and any other instrument designed to play one note at a time) play the melodies and solos.
Considering the evolution of music from being "big band" style music to being jazz ensembles, it makes sense that with the rise of jazz, there was a rise in the popularity of rhythm instruments, most notably the guitar and the piano. Pianos were always popular in music - perhaps the most popular instrument ever, in a way - but during the early 20th Century, they were large and expensive (both hold true today) and only available to people who had sufficient money to buy them, space in the house to store them, and no intention of moving around a lot. These facts explain why the piano was less popular during the 19th Century than they were in the 20th Century, when incomes rose and permanent housing became more affordable. Thus, between rising incomes and the explosion of jazz music popularing during the 20th Century, the popularity of rhythm instruments also exploded.
How Popular Music Was Affected
Well, all these trends occurring in music between 1850 and 1950 had a profound impact on music in general.
The exploding popularity of rhythm instruments meant, first of all, that people no longer needed to assemble a group of twenty-five fellow musicians to start an ensemble and play popular music. Suddenly, it became possible for a drummer, a guitarist, a saxophonist, and a bassist to be gainfully employed and play all the popular songs that everyone wanted to hear. There was rhythm, there was melody, and there was improvisation. They could even ditch the drummer and the bassist and hire a pianist instead. Or play as piano-saxophone duos. All sorts of small ensemble combinations proved viable in jazz music. So long as there was at least one rhythm instrument and one lead instrument at any given time, there could be jazz, which means there could be popular music.
The impact on popular compositions was this: Music started being composed around chord progressions. The more elaborate arrangements proved unnecessary since even set melodies were extraneous in the jazz world. If you had a chord chart and could improvise, you had everything you need. Even if you wanted to play the many popular vocal songs, you needed only a chord chart and a singer who knew the melody.
Perhaps this fact explains why the 1950s represented the effective twilight years of highly composed popular music. At that point, the elaborate orchestration was limited to the vocalists. As the 50s came to an end, popular music took its final turn toward the rock music we know today. The Beatles showed up singing mostly two-part harmonies and playing only rhythm instruments.
But that, too, highlighted a particular benefit of guitars and pianos: They could be employed as both rhythm and lead instruments. In the case of the piano, it could play rhythm and lead at the same time, by a single person. In the case of the guitar, its unique setup enabled people to employ techniques that couldn't easily be emulated on any other instrument: String bends, slides, and so on.
So, by the end of the 1960s, the only instruments most people cared about were guitars, pianos, drums, and electric bass guitars, which were basically low-range guitars played by ex-guitar players. Hence, the core instruments of popular music had become guitars, pianos, and their derivatives. All any band needed beyond that was a drummer and a singer.
The Final Step
The rise of music amplification finalized the process. Guitars became electric guitars, which started to include a wide variety of special effects. These special effects lent guitars a diversity of timbre not available to any other instrument, and all this over and above the techniques available to guitars that other instruments couldn't employ. Meanwhile, pianos and organs became electric pianos and organs, which became keyboard synthesizers. These keyboard instruments also came with their own array of special effects and timbre changes. Finally, even electric percussion sounds became available to keyboardists. By the 1980s, any sound required of a professional rock band was available on just two instruments, plus the human voice. The evolution was complete.
Just as instrument trends drove compositional trends across the big band and jazz eras, into the rock genre, so have instrument trends continued to impact the way modern music is composed. Every instrument has its own unique "flavor," an approach best suited to it. A keyboard player will write music in a different way than a guitarist, and both will write music differently than a saxophonist or a violinist.
It stands to reason, then, that the diminishing number of popular instruments has lead to an increasingly homogenous popular music palette. Today, if you're playing a saxophone, people expect it to be jazz. If you tried to put a prominent trombone track on the next Rihanna single, people would call you crazy.
Non-guitars and non-keyboards are today only welcome in niche markets: You can play a saxophone, if you promise to only use it for jazz or blues; you can play a violin, if you promise to only use it for country or orchestral music; you can a marimba... Actually, no. You can't play a marimba. It's against The Rules.