I would hypothesize that it's because revolver guys tend to be fairly conservative, and while the 9x19 is very efficient in a short-barrelled revo, it's much more associated with the semiauto pistols.
Besides, except for the tricky 547, they require moon clips, which a lot of folks don't want to hassle with. In addition to its popularity with traditionalist curmudgeons such as myself, the revo is often used by a set of folks who are less adept in the use of firearms: the housewife who wants something to protect herself that's not too big or too tricky to operate, the husband who knows he's not going to maintain the gun past loading it and locking it in a drawer "just in case." These folks probably constitute half of the market for revolvers, and they are not going to want to deal with learning to load and unload moon clips. Not when they can just throw .38s into the cylinder and close it . . .