A couple of those (IC/US Title, KotR, namely) were basically due to the fact that Undertaker's been seen for most of his career as a main event guy or as a special attraction...basically, he's never been an "up-and-comer", which are what the midcard titles and the KotR are (or at least were) designed to showcase. Add to that the fact that one of them (HoF Induction) is basically a foregone conclusion. The other two (no Austin, Cena, Rocky at Mania and no MitB) have a lot to do with the timing of his career more than his ability in the ring or a lack of "something" in his career.
wk