Here is the thing. ATT and any other carrier don't really care that much about providing updates to phones that are already sold. You have committed to another two years, what are you going to do? They have already sold the deal, anything extra will be increasing cost. Plus they rather sell you a new phone anyway. This would be an explanation why things are not moving as fast as we would like to see.
Now, let's look at the alternatives. Many people are making their own ROMs, maybe to stick iit to the man, but maybe because they think they can do a better job than a carrier. So, they come up wiht a ROM, people install it, and the ROM gets updated, with bug fixes, new features etc. Then they continue to work on the next ROM. Some of these people work together and mix and match skills, to make the development go faster etc.
Really, it is not that important that ATT comes up with a new ROM, it is most likely already behind the times compared to what is available in the free market.