Sometimes they just don't know. If you breed two snakes together with 8 different mutations between the two of them, and you hatch out a crazy looking animal, sometimes it is hard to know what it carries. I mean, at what point do you start differentiating between whether or nor a lesser super pastel woma spider enchi desert might carry a yellowbelly gene as well? Really the only way to find out for sure is to breed that animal to a bunch of normals and see what pops out.
In addition, secrecy has been part of the reptile hobby forever. I agree, I don't like it and I think it's stupid. However, some people get a kick out of making themselves seem, for lack of a better word, "reptile-1337", by keeping projects a secret. I suppose they also might have monetary reasons to do so, which I also dislike.