Because they are supposed to be 'Minor', not 'Moderately Aggressive', Races. They are prevented from colonizing, I think, under the assumption that the other races who have been exploring into space for a while, like the Arceans and Drengin, have had countless millenia to refine their colonizing ability, whereas the Minor Races are, for the most part, at 20th century Earth technology levels or lower.
Their role in the game is to add flavor/originality and provide a possible trading partner, ally, and servant/slave race (if you are feeling evil and have a strong military, it is quite easy to extort them out of existance). Should you choose to support one, you will have a fairly potent ally - they have the random ability to steal all of a Major Race's technology, remember, and can't be spied upon themselves. You can also arm them with powerful technology and use them to distract your opponent, or simply crush them directly and take their world.
Another thing that should be noted is that, because they don't get colony ships, they build lots of early constructors - they can take quite a few of the local resources in this fashion.
They aren't in the United Planets because they aren't important enough to make a vote needed. It would be like a group of Himalayan hermits joining the United Nations - not powerful, numerous, or influential enough to get a vote. They still do trade because they are 'under the radar', so to speak, by not getting a place of prominence in the United Planets. Who wants to regulate a single-planet trade network when you have the entire galaxy to screw u- er, regulate?
