This has been requested before: /t/6724/1 ... probably more times than that, actually.
Tagging (in the way we think of it) is not standardized in WAVs or AIFFs. Every audio app that reads or writes tags to these types of files have their own way of doing it, some of which are technically invalid, and none of which are guaranteed to work in any other app.
Mp3tag would have to support reading and writing all these different ways; you would be pissed off if it wrote tags in a way that your particular other app didn't like. I can see why Florian doesn't want to bother with it.
That said, for writing tags, I think the more popular method nowadays is to create an "ID3" RIFF chunk in which an ID3v2.3 tag is placed... if Mp3tag only supported reading and writing that kind of tag in WAV and AIFF files, it would be a good start.