Because in Oblivion, you could only become a vampire, and wouldn't it be cool to also become a werewolf? But alas, you couldn't. And that was doubly disappointing because I guess in Morrowind you could with an expansion pack - but I have no firsthand knowledge of this.
In Morrowind: Bloodmoon, you could become a werewolf.
In Daggerfall, you could become a werewolf or a wereboar.
The lore knows many more werecreatures that haven't appeared in games in the same way yet.