Or any other other source of used physical copies of old games - I think I got mine used from Amazon.
I am guessing - GUESSING - that IF Blizzard were ever going to do a sanctioned downloadable version of the first Diablo, they'd host it themselves, rather than farming it out to GOG (or anyone else.) That's just based on how they tend to control their IP in general.
Since, IIRC, Hellfire was not an actual Blizzard release, but licensed to someone else - Sierra, wasn't it? - I would also guess that even if Diablo became available that way, Hellfire might not, because it would be one of those things where you'd have to line up the paperwork and goodwill from two companies, not just one. In general, those sorts of titles have even more trouble getting a digital re-release than other old games do.
QC: Truth be told you aren't missing very much.
And, this.
I know it's sort of a holy grail for some Diablo fans. It's the expansion to Diablo! But even back when I first got it - when I was playing a lot of Diablo, before Diablo II came out - I wasn't that impressed. Yes, it added a couple of new classes, and had new dungeons, but it just did not seem like it was as high a quality as Diablo. Oh, it wasn't utter crap, by any means. But after playing it a few nights, I just went back to playing more Diablo - better polished, better balanced, better tested, more consistently fun.