yEnc was rejected by the MIME standardization group for two main reasons, one good and one bad. The good reason was that it has some encoding pathologies, although these could have been fixed in the standardization process. The bad reason was "it's too hard to add a new Content-Transfer-Encoding because you have to change all the user agents", which given that by that time all the clients were changing to support yEnc it was quite clear that uptake would likely of a new addition would have been fairly rapid.