Just like developers created the problem of having only two major desktop operating systems to choose from because they decided not to support *nix, OS/2, Amiga, Atari, CP/M...
The operating system market is a natural monopoly - the way the mobile OS market has evolved from the early 2000's to now is almost a mirror image of how the desktop operating system market evolved between the early 1980's and the late 1990s. It was always going to evolve into a two-horse race between a mainstream player and a niche player. This isn't "the developers'" fault any more than gravity is Isaac Newton's fault.
EDIT: Interestingly, if you were to time-travel back to the late 1990's and tell someone that there would be two major players in the Smartphone market today, most people would probably guess that those players would be Microsoft and Palm.