Well, we know from more recently that WotC own numbers show that most people homebrew, over 50%. About a quarter do pure homebrew without wanting reference material, another quarter homebrew but like to do things like put Waterdeep or Sharn in their own world, another quarter use the Forgotten Realms, and the last less Tham a quarter use another older published Setting. The Forgotten Realms Adventures hit the middle half, the first quarter don't buy books, and the last quarter only started getting serviced after 2018...Like everything in life, it was almost certainly a massive conflux of factors. Little bit of this, little bit of that. Little bit of the other.
But for Dark Sun's Boxed Set/Adventure sales comparison... it really does seem like it was one DM buying everything.
Though, taking a look at overall figures for 2e D&D: 4.7 million core rulebooks sold over the course of 1979 to 1998... Vastly more books than all the campaign settings so far presented combined.
So maybe it did split the playerbase, some? But it looks like the -vast- majority of D&D players and DMs just didn't bother with any of the offerings TSR put out for settings.