From a programming perspective, DLCs cannot reference each other. Why? Because not everyone buys all the DLCs. If they referenced each other and one of the DLCs was missing, then you've got a game error that could cause bugs or crashes.
Dawnguard gave us a lycanthropy perk tree, but no werebears. The problem? Well, if they release Werebears in, say, Hearthfire, they WON'T benefit from the lycanthropy perk tree. This forces them to either make a perk tree that's awkwardly named "Lycanthropy Junior" or to simply NOT give them a perk tree, which risks making them too weak by comparison.
Dawnguard also provides recognition for werewolves. Dialog options address if you're a werewolf and NPCs react differently if you are. Infact when you first get the offer to transform into a VL, you're warned you'll lose your werewolf status if you do so. The game will not let you be both. However, if werebears were released in Hearthfire, the Dawnguard DLC would have 0 way of knowing you're a werebear because the coding for werebears would be foreign to it; it can't reference it at all. This means the game wouldn't be able to prevent you from being both a werebear and a VL.
Finally, why can't you become a werewolf over and over? You CAN become a VL over and over, but for werewolf? You only get two shots. There's also werewolves in the wild, but only Aela can turn you into a werewolf. Why?
While limiting it to TWO, I can't explain, I think I have an idea why Aela governs werewolf status: because the game could potentially bug out if you join the Companions AS a werewolf and then you go to gain werewolf powers. Could this have been fixed? Yes it could've; I'm not saying it's not doable, but for whatever reason they did NOT make it so werewolves in the wild can give you their disease. The only LOGICAL explanation I can think of is that it was impossible for them to do so without going back and re-touching and re-spec'ing the Companion questline quite a bit. Obviously they opted out of doing so. The result? Well now, if this same approach (not touching the companions) remains for whenever werebears are introduced, then the Companions will NOT react to you being a werebear and there's a potential problem. Thus werebears are a problem altogether.
Those are three ways I see that sort of make implementing werebears at this point an undesirable perspective for Bethesda. Mind you, this is all speculation of course, but I think it's something to consider.