Because it would confuse people not using IE4 - ie, the majority. This is why I think serious work still needs to be done on these attributes. The following URLs point to archived e-mails that cover this subject.
Revision is much more complicated than simple inserting and deleting.
Sometimes you just have your downright overall change. But you still want to indicate a selection of text that has been changed. This is a problem. I currently see no way that INS and DEL can be correctly used because they degrade really ugily. Where it is clear that there has been a change.