To answer the question, "Why so many?" is like asking "what do you want your computer to feature and what will you use it for?
Some of us value simplicity and speed, some like "eye candy" and all the latest bleeding-edge wonders - and to get them they may have to trade some stability and speed. Some want an OS that "just works" and "looks like Mac" or "acts like Windows." Some are tinkerers who want a completely custom OS. Some are just kids who want it pretty and fast for just school work and Facebook and music. Some are researchers or scientists or forensic experts who want a certain set of software for their work. Some are parents who want something for their kids to learn and play on.
I want rock-solid-stable more than bleeding edge latest craze stuff. My computer is very old, so I want an ultralight, speedy OS that doesn't slow me down. Someone else has a supercomputer with terrabytes of RAM and warp clockspeed and they want to push the limits.
Why so may distros? So many different needs and desires of users and different computers to put OSes on. If they all merged into a one-size-fits all OS, it probably wouldn't run on my old machine, or it would be slow enough to frustrate me. Different OSes for different requirements, different tastes, different hardware, different applications.