Well some people don't want to open up their machines every time they wanna play a 4-player game. Plus lots of people don't have 4/8 way controller that really works the way it's supposed to. The Happs 4/8 ways are a joke. Since most people buy Happs 8 ways they need a dedicated 4 way.
If you don't mind opening the thing up every time you wanna play a 4-way game, then by all means gor for it.
Plus lots of people prefer bat-style joys for their 8 ways and ball-top joys for their 4-ways.