Maybe, saying "twenty" isn't enough of an improvement over "two-thousand" to make it worth the complexity. Compare that to "one-thousand-nine-hundred" vs "nineteen".

Also, years less than 10 years ago which are still referenced a lot then require a switch to a different reading convention. This, I guess, wasn't a big deal from 1999 to 2000 because, "twenty-oh-oh" was too awkward and, hey, it's two-damn-thousand!