UHT milk hasn't caught on because it tastes only slightly better than reconstituted powdered milk. It is, however, cheap, and hasn't had trouble at all catching on in poor countries.
In Europe, UHT has caught on even in "wealthy" countries. In most shops you can buy both kinds, the UHT cheaper and the unprocessed as the more luxurious version. I think you're overstating the taste difference — as a child, I was often drinking really fresh milk (my grandma had cows). Later, most of my teenage life I was drinking UHT, and now I'm drinking the unprocessed refrigerated milk. I agree with you that the unprocessed one is better, but the difference isn't mind-blowing. I even know people that prefer the UHT taste.
