The thumb throttle does a better job of isolating your arm movements, then a wrist throttle. Yet very few dirt bikers use a thumb throttle. So I really think it's what people prefer, and what they are used to. read more
Thumb throttles are in a word "safer". When the early ATVs were being developed they copied the snowmobile throttle because if you loose control of the bike, you are less likely to grab/roll on more throttle with a thumb throttle. read more
I have ridden a dirtbike with a thumb throttle before, and it sucked ass. The inability to fully grip the right bar makes riding more difficult, IMO. Also, steering a twist throttle four-wheeler kinda sucks, because of the range of motion of the handlebars. It makes throttle control more difficult. read more