Yet they were not in Oblivion or the expansionless Morrowind, also Werewolves would take far more effort to include than Spears and Crossbows.
No, but lore isn't centered around werewolves exept in Solstheim and Skyrim. They do exist somewhere in Cyrodil and Morrowind, they're just so rare that if Bethesda wanted to program a percent chance of encountering a werewolf in either game, it would be so small only a select few people would ever find them. Skyrim is the were-creatures home. And yes, they would take effort, but so would everything else in this game. It would not take as much effort as having all items 3d, which many people hardly care about since when you equip them on your player, they are already 3D.
Oblivion and Morrowind were not supposed to have werewolves. Skyrim is; leaving them out would contridict lore and more importantly be a waste of oppertunity. Werewolves don't take much more effort than Vampires.
Skyrim is newer than Oblivion and Morrowind so it would be even easier to encorperate them.
Adding Vampires but not Werewolves is just a terrible idea in general, since Vampires are supposed to be less rare in Skyrim than were-creatures.
I have another theory on were-creatures: They are saving them for some kind of DLC.