As difficult it is to get Arena and Daggerfall to run properly on modern systems, Battlespire and Redguard are even worse. And since GoG requires that any games sold on the site be XP and Vista compatible, the amount of work to get them into that shape would probably negate any money they got from selling it.
I know they use DOSbox for games like Duke Nukem, stonekeep etc. Gog does the actual tweaking, although who knows if it's worth their time. It's from a popular series so it would get a fair amount of sales... at least from people interested in these 2 relatively unknown elder scrolls games.
Also, I must be lucky for some reason. I've gotten Arena and Daggerfall both to work fine on vista and the only issues I had were my own mistakes I fixed quickly. *shrug*