Why isn’t this more prevalent?
It's mostly a matter of practicality.
It's difficult to build small greens anymore. If you are building a course that expects a lot of play [which most clients hope for], the superintendent is going to insist on at least 5000 sf per green, possibly 6000. So the lower bound is more limited than it used to be.
And if you are thinking practically, and 6000 sf will suffice, it seems pretty unnecessary to build one that's 12000 sf.
Sure, you can find ways to justify it, the best of which is "that's just what I felt like building". 
But the number of times I've worked on a hole where I thought it really NEEDED a 12,000 sf green are small enough to count on one hand.
Variety is important. Overdoing variety for the sake of publicity is not the same thing.
I do like the variety of different size greens, but it can be between 4500 SF and an upper limit of maybe 10K.
The design (as opposed to marketing) reason I have often heard is that a large green on a shorter shot "might" make one lazy on the approach, which is certainly a different way of trying to fool golfers. Perhaps that large green could be partially hidden, or even have large bunkers to fool distance perception. The other justification I have heard is that if you have one green where the penalty for an indifferent approach shot is a monster putt rather than hazard, there is nothing wrong with that, although many golfers reject the idea of a shot you really can't practice for, and who wants to check with their medical insurance on a shot? (A 200 foot putt takes quite a hip turn!)
Having been in the biz since 1977, I will say it wasn't very practical then to build a green under 5K, so it is (to me) a "now days" type issue. Similarly, supers usually balk at much under 6K and anything much over 6500 SF, which is difficult but suitable to find enough pins. Is building 4% contours and other wild internal contours within, all of which require bigger greens than the acceptable minimum also a way to justify bigger greens? For most golf courses struggling to make money, that answer would be no, but I see a lot of architects following those trends, which I believe will be found as impractical in the next recession, as wild features that are harder than standard to maintain have been. I used to think (still do) that except at high end courses, anything that is difficult to maintain will be altered within a few years from opening.