Remember ST is a TV show and it has to be produced to a budget, as for it always being sunny, isn't that more a case of were the show was filmed.
Was there even one episode of TOS where it rained--on Earth or an alien world?
Not that I can think of, off hand. Los Angeles is generally a dry region. Of course that doesn't mean they couldn't have simulated rain on the soundstage, but then it would probably mess up Spock's eyeshadow and Kirk's "Hair"!
Plenty of myst and wind, though.
I can scarcely think of many episodes where it Rained in TNG or DS9, either. It happened, but It wasn't all that common even in later trek.