Simple question of economics. Why pay $15 for virtual copy when you can buy a physical copy at Gamestop for $5? Also, the lack of HD space probably had a lot to do with this too IMO.
http://kotaku.com/5294582/microsoft-...xbox-originals
I bought GTA: San Andreas just to have the ability to play it directly from my hard drive anytime I wanted to. As a lifelong PS2 GTA player, the Xbox's superior visuals (720p) were a nice bonus.