that's like asking why the other 23,768 classic PC games that should be on GOG aren't...
GOG has a relatively tiny range of games on offer. 380. now look at Steam: just under 1800 last time i checked. and that's nothing. now look at GamersGate: some 3000 games. and still you won't find hundreds of must-have classics there.
there are dozens of all-time classic series that should be on GOG but aren't: Civilization, Doom, Age of Empires, Command & Conquer, WarCraft, Need for Speed, Comanche, etc. etc. the list is endless and chances are most of them will never show up.
SimonG: Actually, id was bought by ZeniMax who also own Bethesda. Not by Bethesda itself. And ZeniMax also owns Arkane Studios, who is on GOG.
So, there is hope left...
right, but there isn't all that much difference between ZeniMax and Bethesda, at least not outside the legal sphere. both were founded by the same guy.