Runequest 3 killed a lot of the fanbase for RQ in the 80's and 90's. I don't know how bad it was, but the current Runequest edition is based on RQ2 so that's considered the peak by a lot of grogs. Avalon Hill were also more used to releasing board games and not RPG's, they didn't know how to release and market those. Then Avalon Hill shut down and yet Chaosium didn't get the rights back, so for the 90's and early 00's RQ was pretty much dead. That's a big start, beginning of the end. RQ3 was disliked, then copyright shit prevented RQ from even existing for a while.
Though they still had a fanbase, I really like looking at old 90's RQ/Glorantha. A lot of those zines bemoaning the death of RQ were made when the third edition was still new. So much Runequest fan stuff comes from this era and it was global thing, US, UK, Japan etc.
Then the 00's, there was Heroquest, not really Runquest but it was still connected to Glorantha and original Glorantha creator, Stafford, but doesn't use the BRP system. And then Mongoose's RQ, the IP kept getting passed around owners.
RQ6(which would become Mythras) was pretty well liked, then Chaosium got the IP back in the 2010's and now we have RQG/RQ7/RQ4(the numbering if you ignore anything with no Chaosium involvement) which is the current edition, and the last one with any involvement by Stafford or Steve Perrin. So Chaosium has RQ back, the people running it are old fans who want it thrive, and they're trying to push it, even in Japan where it used to have a playerbase.