You could fairly easily answer most of your own questions.
Firstly, WHY do people set up generic faction, KitPvP, etc servers?
Because it has a large playerbase, which means it generates traffic quickly, which means that it generates bucks.
Secondly, WHY don't people set up unique servers?
Because it's not easy. If you want to set something unique up you'll most likely have to know how to code. Hell, my own custom server's plugin inhabits over several thousand lines of code. All of that takes time, energy, money for server upkeep. It's a lot easier to just plop *insert generic plugin name here* into the server and let it do it's thing.
That pretty much outlines the whole reasoning behind MC's server variety's state at the moment.
