I think there are a lot of different reasons why (just a SLF speaking by the way so please correct me!) such as...
- Size of aircraft (i.e. 747 needs longer to stop than a 737)
- Different styles of exits (i.e. high speed exit angled specifically away from the runway compared to a 'normal' 90 degree turn)
- Following traffic (i.e. need to get off the runway quickly)
- Fuel burn (presumably these days you would be looking for the shortest distance to the stand)
I remember landing at Edinburgh with Nortjet (back in the day) and in my opinion we were coming in quite fast. There was one exit from the runway about 2/3 of the way down and if you missed that you had to go all the way to the end, do a 180 and backtrack. The pilot must have realised that after landing as there was some impressive braking, some interesting noises from the brakes (I was sitting above the MLG) and everyone leaning forward with the deceleration. Dunno how much extra fuel he would have used if he had missed the turn, but I guess that's a reason too - $$$$$$$