110mH / 400mH runners - why so few?

For the same reason you don't see too many people running and succeeding at the 100 and 400 at the same time (notable exception, Xavier Carter). Different energy systems and skill sets.

BUT, more recently, we have seen several NCAA athletes (women mostly as I'm remembering) have success in both at the same time.

Queen Harrison at VaTech, Anna Cockerel at USC, and just this year Masai Russell at Kentucky.

Hurdling technique is also rather different in 100/110 and 400 m hurdles, so the difference is even bigger than between 100 and 400 m flat.

two meme events on the same level of racewalking

just look at the last countries who won 400h: Norway and Brazil. Two t&f powerhouses.

Stud 400hh Andre Phillips was a 13.2 110 who tried out for the 1988 in the event in 1988, he came in 4th.

Because there is a big difference in those events.

The former event requires insane mascularity and top speed, the latter requires slightly lighter body that can go on and on

The guys you cite above disprove your point. The first two wouldn’t make an NCAA final these days. The third guy achieved those bests 5 years apart. There are, of course, many athletes who COULD hit good marks in both, but the training between the two is less and less complementary the better one gets. So staying in both has diminishing returns. The pool of talent is both wider and deeper than the era you cite. So while Grant Holloway is certainly talented enough but would likely not have been a global champ if he was spending time on 400H