The technology isn't thought to be mature or sufficient enough that many armies bother with upgrading their tank fleets- so far a system like that might do well under select conditions, particularly against one or two insurgents with an RPG or an ATGM. But against a conventional opponent or in high intensity conflict, it's difficult to test a situation like that, it would essentially be an untested system going into battle, which is why there is still more attention focused elsewhere improving the survivability of armored vehicles. The Russians have also studied ways to defeat the sensors of these kinds of defense systems by getting them to fire prematurely, so of course there are responses to every sort of technology that is introduced.

Likewise, I would imagine that even if railgun technology advances sufficiently so they could fire multiple times without damaging the rail itself, there would still need to be a compliment of surface to surface missiles, air defense weapons, and so on. It would run the danger of putting all your eggs in one basket otherwise.