Because the people in control of the game really hate clowns for some reason.
First clown got removed entirely, then it was begrudgingly allowed as a special day job, but then traitor clown cars got removed on top of that.
I remember when being a clown meant something. Granted, I wasn't around when it was originally meant to be a punishment job, but I remember station clowns being whimsical jokesters who would gallivant around the station falling on their faces, doing merry jigs for the crew, and getting killed shortly after showing up. Any clown with any modicum of integrity wore the full outfit at all times, even in the face of depressurization. And there was only ever one clown at a time, because really you only need one good clown.
A traitor clown was more hilarious than dangerous. Granted, security was every bit as likely to converge on them and murder them on the spot as they would anyone else (it was a different time and sec was far more bloodthirsty than they are now (SHOCK, I know)), but they really couldn't do much but throw banana grenades and abduct people in their dumb little car, which was more like forcing people to go on a grand adventure with you than any actual harm (unless the dummy drove out into space in which case he was just as screwed).
The thing is, you have to be wearing full clown gear to use the car. The only problem I can figure is that since more than one clown suit can exist now, that means anyone who wants to can basically drive the thing, instead of having to steal a clown's suit right off of him to do it. So I guess that's the most likely reason it was removed. I can't imagine there isn't some other way to do it, though, like making you have to have the dyspraxia gene (the clown clumsiness gene) to pilot it?