I take the bus from Korat to Cha Am a few times a year to visit an old from who has a condo there.
The buses from Bangkok to Cha Am really suck: the air con is no good, the shocks went bad a decade ago, there's no leg room, all they serve is a bottle of water and, other than your fellow passengers there is no entertainment whatsoever. This is sad because the shocks are so bad that you can't really read or even enjoy the outdoor scenery.
On the other hand, the buses from Korat to Bangkok are pretty good: decent air con, shocks that actually work, snacks, water, Pepsi, entertainment (well, Luk Thung music videos) and a whole lot more leg room. They even give you one of those little packaged towels when the trip is over.
I find this ironic because the Cha Am area is a big tourist destination and there are always other farang on the bus suffering with me as I make the dreaded three hour trip.
I know about the mini-van and I may take it back to Bangkok, but I've heard so many horror stories about it (bad driving, lots of accidents) that I'm not really sure it's an option.
Anyway, the main question is: why do the buses to Cha Am suck so bad?