Well, it's not like they have a lot of options. Consider the powers that surround them -- the Klingon Empire, the Romulan Star Empire, the Cardassian Union, the Ferengi Alliance, the Tzenkethi Coalition, the Breen Confederacy, the Gorn Hegemony, the Tholian Assembly, the Talarian Republic. Frankly, the Federation doesn't have a lot of nice folks to make friends with -- the closest would be the Ferengi Alliance.
It's one of Trek's bad production decisions: the idea that if anyone out there was going to be a good guy, then naturally they'd be part of the Federation. It's fine for the Federation to be the United States Writ Large, but its existence doesn't mean that the United Kingdom, France, and Brazil wouldn't also exist.
More like the European Union Writ Large, if you ask me...
But yeah, I think that was a bad decision as well. It would've been nice to see some allies that didn't get themselves assimil--er...um...absorbed
into the Federation.