Because they are being used? (In turn because they tend to be the best ones around, most of the newer email software tends to be crap if you go by productivity rather than shininess.)

Also, this is not just about compatibility with old clients, but also about compatibility with old emails. If you redefine the meaning of previously used email syntax, that changes how old emails get displayed, which sucks if you have an email archive that goes back more than a decade.