Its pretty impossible to make a decent 6*+ map without 12 34 mash patterns.
Star rating is broken anyway because it looks on density (aka mashing) instead of difficulty of patterns. For example Risk Junk - One More Lovely x1.5 (ATTan) [Challenge(20)] at 1,5x rate was only a "~5,5*" what is funny.
EDIT: You can always try to map it yourself and see how easy is to make a decent and rankable (without stupid dump stuff) 6* map.
Most insanely hard files from SM barely scratch 6* unless they're AAable for only two people - unless they're filled with walls or some bs that not many people will enjoy. I know Japanese Transformation (Fullerene's) is going for rank and that is 6.31, that's the only map I know that might have a chance of being ranked. M.A.M.A. counts too I guess.
Even those two maps I don't even consider 6* material, neither map are any harder than IN2006 (which is rated 5.3, when it should be something like 5.9-6.0) or AiAe (which should be justifiably rated something around 6.1-6.2 rather than whatever it is atm). I think JPT should be something like 5.8 and M.A.M.A. should be significantly lower than it is atm. The difficulty system we have now is severely flawed in the upper echelons (of 4K gameplay at least), but I think you already knew that. That being said, 6 star 4K maps probably aren't going to be ranked any time soon because of myriads of reasons, like song choice, a lack of understanding of how to make justifiably hard maps and so much more.
I personally don't care about star rating though, more difficult 4K maps would be nice in general - but I can understand why they're not as common as something significantly easier. Will have to wait, I suppose.
the lower the amount of keys, the lower the skill ceiling is there are only a small amount of patterns you can use in 4k, so the only way to make a 4k map with higher star ratings are to increase the speed (increasing density) at some point, those patterns become unhittable