I remember an ENWorld thread many years ago, I think it was started by a publisher, which was called something like "why don't you buy published adventures?" And by far the most common response was "because I can't fit them into my campaign". Published adventures, at least if you read that thread, were all for level 11 characters adventuring in Illusk with a cleric of Azuth and a halfling ranger in the party and they featured a nine page introduction describing what the characters did at levels 8-11. What GMs wanted was an adventure for adventurers at level 10-12, with an introduction of "the party are looking for a missing paladin (reasons left to GM)".
Of course, this was exaggerated; but I thought there was a fair degree of truth in it.
After that I noticed an uptick in material that solved the "doesn't fit" issue. There were short modular adventures that took place in "a ruined castle in a secluded valley", that would plug into any campaign that had valleys and castles. And at the other end of the scale we saw adventure paths, which didn't have to fit into the campaign because they were the campaign. And times were good, and adventure sales appeared to pick up.