Throw distance, speed, prisms effects, macro effects, physical size, cost... There are plenty of reasons to choose certain fixtures for certain areas/purposes. Sometimes you need something with a high output and a far throw that doesn't need to move fast, sometimes you need something small and compact that can throw light around fast but don't need a huge output. Other times you need a zoomable fixture with shutters and CMY. The list goes on... other times you have X to spend and need Y fixtures, so you divide and see what you end up with.