Speaking from personal experience, sometimes it's just about what you're currently into.
In my case, sometimes I wanna play the generic son-out-to-fuck-his-HUGE-titted-mom type of game, other times I just want a good story with rich characters and diversity in appearances.
As for how devs develop their games, I can't speak for everyone, because it differs from one dev to another.
Some devs create their games based on their own fantasies (refer to line number two), which isn't a bad thing, but it also isn't good for the whole "get your game out there" type of thing, as there are many other games like that.
Other devs, who in my opinion are talented and skilled enough to keep their desires and fantasies out of their work, try and go for something new and unique in its own way.
I'm not saying I'm a good dev by any means, since I'm fairly new to the whole thing (two months old or something), but I tried to go for that last bit in my game:
Screenshots from my upcoming release:
-Diversity in breast sizes:
-Diversity in ass sizes: