IMHO different classes, formats, and cars come and go, as people migrate from one to the other - some seeking a class or a car with which they can win at (everybody gets into this hobby wanting to be the next champ), and others just trying to find a niche in which to fit, while others migrate from one class to another buying each new release (and next hot thing) that hits the market.... it's seemingly always been this way, and I've seen nothing that suggests it'll change.
2wd was really the first class in electric RC (with a history going back about 30 years), and 4wd first came along about 20 years ago (with it's high and low points over the ensuing years).... this is not to say one is "better" than the other, but just saying the market is more consumer driven than anything....
"if you keep buying them, the manufacturers will keep building them" 