Your anecdote doesn't match mine.
Not trying to dispute your personal experience, but
really? Your friends would
only buy books for one setting, and refuse to buy any books from any other setting?
That just seems truly bizarre to me. Among my friends each of us had a
preferred setting that we would focus on (and use when that person DM'd a campaign if they didn't want to use their own homebrew world) but all of us would happily buy any book from any setting that caught our interest (we had 8 people in our core D&D group of friends and out of those 8, 1 was our primary Dungeon Master who ran our primary campaign, but 4 others also routinely ran our own long-term campaigns as well, and 1 other who also ran a short-term campaign and the occasional one-shot games when the feeling struck him. At the time I just thought that was normal for D&D groups, but from what I've heard online since then I guess it is highly unusual?) The only reason we didn't all buy up every book TSR released was simply because there were
just too dang many for us to afford, and absolutely not because we didn't want books from different settings.
I suppose we could have just been a very weird group of people, but all of us had collections of books from just about every setting released; one friend focused on Dark Sun but also had plenty of Dragonlance and Forgotten Realms books, and while I focused on Greyhawk as my favorite setting I also had as many 'Realms, Ravenloft, Dark Sun, Birthright, and Planescape books as I found interesting and could afford (I even had the Tales of the Lance Dragonlance boxed set despite disliked the setting; even though I had no interest in the world itself there was still plenty enough good material and interesting ideas that I could mine from the boxed set that it was worth having.) Our primary DM focused his book collection on the 'Realms, but almost exclusively used his own world for his games and had an extensive smattering of every other published setting, including Mystara and Birthright. Also, this was despite the fact that our group used
1st Edition (aside from when we played our Dark Sun campaign simply because Dark Sun was custom made for the 2E rules and so many basic rules of the setting were based on the 2E rules, especially psionics, that modifying things for 1E just seemed pointless.) And it wasn't just our lone gaming group that did this, either; every D&D player I ever personally knew did the same, owning books from multiple settings, and I seriously never heard of a player refusing to buy a D&D book just because they were labelled with a certain setting's logo. If a book caught their attention and seemed interesting they would buy it, regardless of the setting logo underneath the AD&D label.
But maybe Central Pennsylvania is just a weird outlier in D&D buying habits... it certainly is in many other areas of activity. But if you hung out in Comic Swap and Nittany Line Hobby across the street from Penn State Main Campus in the early '90s, you would routinely see people buying up piles of Forgotten Realms, Dark Sun, Ravenloft, Spelljammer, and Planescape books all in the same batch as a matter of course.