Actually a large percentage is about the money but there is local community standards and ordinances that come into play as well... It's all tied to money but just the same...
Here is a perfect recent example
Australia Bans R-Rated Movies Without Banning Them
Also depending on community standards R rated movies can be a hassle for the theater, the last theater my my was a projectionist at before she retired was in a 'ritzy' area, the town past an ordinance that they had to card EVERYONE purchasing an R rated movie ticket just like they were purchasing alcohol or cigs, no ID no admittance... I'm sure it's not the only theater that has had community standards imposed upon it making R taboo...
Yeah it all boils down to money in the end, it's the same reason you don't see many (non-porn) X rated movies...
Interesting to look over the list here
X rating - Wikipedia, the free encyclopedia and see what movies were originally given an X and knocked down to R, the same thing is happening now with R being knocked down to NC-17 or PG-13... Just imagine if all original Friday the 13ths, Scarface, RoboCop, and Total Recall had all been released with their original X rating, it would have been disastrous for them...