I was one of the reiserfs users before it happened and I dumped it quickly. I think an awful lot of people did the same, and since it was one of a number of alternatives to ext I think it is more abandonment than obsolescence.
Now I think my decision was wrong headed, so long as he was no longer involved. The NASA example was given. Alfred Hitchcock movies? Abuser of women but damn good films.
reiser4 was a promising filesystem (had transparent compression very early on) but reiser3 (reiserfs) didn't really have anything significant over ext3/4 so abandoning it made sense. And reiser4 never made the kernel so it's understandable that most never bothered with it, although I used it for a few years before btrfs became viable.
At the time the competition was against a nascent ext3, as I recall, against which reiserfs had significant advantages. Journaling wasn't standard back then and wasn't handled by ext2, so there was a lot of competition, and it wasn't clear that ext3 would be the best solution.
Now that I think about it ext3 wasn't even a thing. And when it did come on line reiserfs already had a mature journaling system.
Reiserfs was the first journalling FS included in the Linux kernel. Ext3 didn't make it in until 2001. So reiser was the only game in town for us and why we moved that way. By the time of the murder ext3 was mature so the switch away wasn't a big deal.