I would need to see some data on the subject to come to a conclusion.
But I would guess that Bitcoin gambling is still way less popular than traditional gambling in casino's, thus there being less high rollers in online Bitcoin gambling.
I agree, bitcoin gambling is probably taking less than 1% of all wagered amount among traditional and online gambling. That's why high rollers are not so common in bitcoin gambling space.
On top of that, it's much easier to identify high rollers that gamble with Bitcoin on dice sites, because they usually show their stats on the website.
This is not the case with physical casino's, people could be spending hundreds of millions of dollars and people would just not know about it.