As the game progresses and level caps grow, high end relevant content continues upwards in level and gear.
This is entirely natural and important to games to prevent them from stagnating, but it does have the effect of causing older raids to become less relevant and less frequently run.
This in and of itself is not that big a deal except that there are still often just one or two pieces of gear people like to have out of these raids either because they are useful or fun.
Nowadays, even Shroud, the ultimate twink gear raid with min level 12 yet very powerful gear, which has for a long time been by far the most commonly run raid, has started to become more uncommon to see LFMs.
I suggest that as raids become older and more out of date, hence less frequently run and harder to find groups for, the 20 completions should be scaled back. Perhaps dropping the few with level 20 epic versions down to 10, and those without down to 5 or something.
Or perhaps a more consistent approach like minus 2 per update that has come out since its release to a minimum of 5 or something.
Either that or make the 20th completion much more comprehensive a list, and make it so every 5th completion or so would spawn 2 or 3 of the possible named items.