The related question discusses what kind of learning is needed to bring forth a kaddish derabanan. In many minyanim, some learning (or a two-minute quick drash by someone) is undertaken between mincha and ma'ariv and is followed by a kaddish derabanan.
But I have not seen the kaddish derabanan said after a sermon by the rav on a Shabbat morning. The content certainly is equal to or more comprehensive than a short vort in the evening (and one could certainly add the "R. Chananya" passage).
Why hasn't having a kaddish derabanan become instituted at that part of the davening?