There is a few reasons I can think of,
Gearing, it allows you to run a nice 18-15 gearing small bashring and huge clearance.
Weight, sounds silly at first but it gets the weight of the freewheel into the middle of the bike and it actually makes a difference the back end is just the bit more flickable and weight distribution more even for balance
speciality components is the last reason, trials has gotten to the point where things are actually being made for us, all this stuff is front freewheel and it's actually good stuff that works well, freewheels like the try-all and Echo SL are solid and have masses of engagement so why wouldn't you use them?
