From the Straight Dope.

Back when Cuba was under Batista and still a wretched hive of scum and villainy for depraved Americans to get their kicks, there was a nightclub performer, as well-known as he was well-endowed, who went by the moniker of "Superman" and specialised in live sex shows. He gets a fair bit of mention in the literature of the period, including Graham Greene's Our Man In Havana.

Simply put; the subject matter and the way it's presented.

The film is a slapstick comedy that is chock full of sexual innuendo and racist jokes. Keep in mind, this was before the "PG-13" rating was created, so if it was rated PG, kids could go see it. And it's clearly not intended for children. Fart jokes aren't grounds for an R-Rating.

This is a classic plot of westerns. They absolutely could re-route the railroad. But this would likely be costly. It was likely cheaper and faster to drive people out of their town, buy the land cheap and tear down any structures.