Lots of morphs come in every year. Pastels, hypos, yellowbellies, cinnamons etc. Just because you don't see it advertised on a public forum doesn't mean its not there. There are some morphs (even dominant) that there have only been 1 or 2 found. It's not like there are morphs just hanging out everywhere, all over Africa. The people in Africa look all over the place for new morphs, as they know where the money is at.

A lot of people import them, but you have to have particular licenses and paperwork in order to import animals into the US, which is why a lot of bigger breeders are able to do it as opposed to someone wanting 5 or 10 snakes to "take a chance with". Its not worth it cost wise for someone to bring in a few snakes over a few hundred.

Trappers will contact breeders in the US when they find something particularly different though. The normal public won't ever hear about this, nor see them until later, once the animals have already been imported and are in someones collection.