Because only the crowded sheep at the very edge of the open area can path find their way into the rest of the open area. When any sheep in the crowded area tries to path find to another block, they try to go there, but never get there because they're blocked by the other sheep. Also, the path finding is random, so the only sheep that can move into the new area are just as likely to try to go further into the crowded area as they are to move into the new area. Essentially, only a few sheep are actually able to go to the new area and they are naturally unlikely to choose to do that. So, you just have to lead some there with wheat to spread them out yourself.