Leyzer:- I was under the impression that they were supposed to be giving over Halocha?
Thou art One Thousand % correct in your assertion, and even Shabbos Shuvo Sermon is supposed to include some Hilchos Yom Kippur & Hilchos Sukko, in addition to being about repentance.
Once the Rabbi has covered his minimum obligations towards disseminating Halocho, he can follow fashion & do whatever he [or his Board of officers expects] wishes, and most Rabbis want these 2 Shabboss sermons [Shuvo & haGodol} to be different in format from a regular speech.