Well, anybody of any political persuasion can figure out smoking pot is fun. But there is a real tendency for Republicans to not be potheads and for potheads to not be Republicans. It's not an absolute, but it's a correlation that needs explaining.

I think Republicans are mostly against weed because they tend to be very Christian and very patriotic. A strong obedience to church and state creates a state of mental dependence based on fear, causing a lot of people to blindly follow tradition and authority when it tells them certain things (like weed) are taboo. Republicans are less likely to have exposure to weed smokers, and are less likely to try it to begin with. One major factor in potheads' distrust of Republicans is the fact that Republicans have been participating in the screwing over of potheads over for the past 70 years.

Also, the pot culture puts a big emphasis on sharing weed. You smoke other people up, they smoke you up, it all works out in the end. This clashes with the Republican advocacy of cut-throat greed-driven capitalism.

There are also cultural factors that drive the negative correlation between Republicans and stoners. The very left-leaning hippie movement contributed immensely to the popularization of cannabis in the US. Hippies tend to be on the left, and people who hang out with hippies tend to be on the left, and children of people who hung out with hippies tend to be on the left, and friends of children of people who hung out with hippies tend to be on the left. As the cannabis culture grows, so do left-wing ideals, because of the continuing influence of the hippie ideology. It is often very watered down hippie ideology, sure, but it's still enough to make stoners less likely to pick the Republican candidate on the ballot.