I guess the reason why they want to keep the cap is a technichal one. It is probably not possible or really hard to remove the cap in class Wc3. And they cannot increase/remove the cap in Wc3:Reforged and also implement balanced cross compatibility between classic and reforged.
It would probably be possible to increase the limit slightly (18 seems realistic, as that would only require minor UI changes in classic Wc3), but a change like that would not be enough to please the players who want the cap removed - so what is the point.