Well, if I'm not mistaken, it was the Bajoran government that wanted the Federation help, and the Federation wanted to help Bajor in the first place (but they couldn't do anything until the Cardassians left).
Furthermore, I agree that Bajor was a viable candidate among other things because it's close to Cardassia - however, we've seen the Federation being interested in less developed planets for example and admitting them into the fold.
Most of all, I think that while a world has to meet certain standards to join the Federation, they have to have certain cultural issues settled.
The caste based system was done away with by Bajorans themselves a long time ago, and it's re-enactment would have meant no entry into the Federation, so from a cultural point of view and social standards, I think that Bajor passes most of the parameters, and while religion is a big part of many Bajorans lives (though not ALL of them), one has to keep in mind how that perception could have persisted due to the Wormhole aliens excessive meddling in the early days, but later on when the Cardassians came and effectively enslaved them (they turned to the only thing that played a big part of their lives - but for all we know, they could have been moving away slowly from religious perceptions of the Wormhole aliens as they continued to develop technologically, but the occupation brought it back in full force).
Other than that, while I am annoyed by religion overall and steer clear of it (along with concepts such as 'belief' 'faith' 'truth' etc. in any shape or form), Bajorans seem to not have any major internal issues or use their religion for harming others (for the most part) or starting wars.
To that end, we know the Federation enjoys providing relief effort to other species if they need them, which can easily pave the way for better relations down the line.
Also, Bajor was not admitted into the Federation immediately.
It took several years after the occupation ended, and while the Federation provided outside help, Bajorans did manage to recover fairly quickly (despite the hiccups).
I would imagine that many of the Cardassian and Bajoran technologies left after the occupation was streamlined to work with Federation tech, and that probably increased the speed of recovery.
And entry into the Federation was again delayed because of the Dominion War and the way how Ds9 completely made retards out of the Federation (among other things) as the show progressed.
