Because there’s typically 4-6 months between tiers. Guilds have a certain amount of downtime.
You used two examples for comparison, Highmaul and Trial of Valor.
Highmaul was the entry raid. Of course it was going to have high participation rate. It was that or do apexis dailies, as there wasn’t even mythic dungeons yet, much less m+.
Trial of Valor was the first raid of the expansion that didn’t fall over dead. EN was infamously cleared in less than 18 hours by top guilds, and for more low-ranked, what would normally be a month-three month progression was closer to 1-3 weeks. So, again, guilds were chomping at the bit for something to do aside from farm AP and legendaries.
BoD was a notably easy raid, pre-Jaina, for higher ranked guilds. It’s been jokingly called a 1 or two boss raid, Jaina, and the other 8. The rate of progress is roughly similar to Uldir, with the major difference being, Uldir bosses had a more gradual difficulty curve, whereas BoD spikes suddenly at Jaina.
So guilds finally kill Jaina, and a two-boss raid comes out that’s expected to be relatively easy, as a way to simply pass the time for high ranked bored guilds, and to acquire some extra/special gear for lower ranked guilds still progressing BoD.
Then, horror stories start spreading like wildfire. Method, Limit, Pieces, and other top of the top guilds mass-stacking a dozen spriest (which, funnily enough, never actually occurred) and still wiping for hours on end, non stop. More wipes than Kil’jaeden, the previously undisputed hardest ever boss.
Guilds that hit CoS when it opened quickly decided it was not worth the strain. Others that came later, were scared off by the hell that was Method/Pieces progress.
Thing is, after all the nerfs to both bosses, they’re not nearly as bad. Example, Method and Pieces were progressing Uu’nat pre-nerfs, and finished with over 700 total wipes. Limit, who only progressed post-nerfs, finished at only just over 200 wipes. Comparatively, post-nerf Uu’nat is relatively easier than pre-nerf Jaina, by 100-250 pulls, depending on your raid.
The loot is objectively strong. Sims and representation in top logs both show that. Some have exceptionally strong use in m+ and, as noted, PvP.