AV stands for "Anti-vehicle". A dedicated Anti-vehicle player sacrifices his anti-infantry capacity for the ability to destroy vehicles. His fit is ONLY useful against vehicles, and by definition should be effective ONLY against vehicles. If another asset provies you with the ability to destroy vehicles AND infantry, and with greater effectiveness (and similar cost) than a dedicated anti-vehicle infantry unit then naturally you will gravitate to that asset, especially if your anti-vehicle capacity is ineffective, as is currently the case. This is the creation of a zero-sum game and this is what is occuring now.

I've translated this analogy to make it dropsuit-centric so people can grasp how serious this issue is here:
You have a heavy suit, that can kill other heavies AND all other types of dropsuit - it can be countered by a light suit, lets call it the "anti-heavy" light suit. Now the "anti-heavy" light suit is not really effective on its own against the Heavy suit, even though that IS it's intended role, BUT it can also be killed by every other suit that exists, and it's not at all effective against other suits.
To take down a heavy suit you need two or more of these "anti-heavy" light suits to work in co-ordinated precision, timing their shots, taking cover, reloading asymetrically to keep the pressure up all the while avoiding the fire of the heavy suit AND other suits (again, from which they cannot defend themselves effectively). After doing this for a while, the players running "anti-heavy" light suits figured out that they could run a Heavy suit, which is more effective at killing other heavy suits than the "anti-heavy" suit AND they could engage all the other types of suits, without even resorting to teamwork - it's from here on out that everyone in the community came to the forums to complain about "Heavy suit spam" - since in every match it was not uncommon to see up to 7 heavy suits per side, completely dominating the map and making all other suits irrelevant.