This is the answer. Most people don't run any particular dungeon for the challenge or the fun, at least not after the first few times. Most players run dungeons for the rewards, and the rewards in the dungeons below level 50 aren't worth it for most players. The only people you'll see queuing for any specific dungeon (barring the current speedrun flavor of the month dungeon) are people who need it for some reason (GC hunt log, quest, animus book, glamour), or people tagging along with those people. Or, very, very rarely, the special snowflake who honestly just feels like running that particular dungeon for one reason or another. Anybody else is there because it's either their roulette or their highest available dungeon while leveling.
And for people leveling, it's always bottom-heavy. There are far more people with Sastasha unlocked than with AV unlocked, and so on. For every one person with only Sastasha unlocked, there are a few more people pulled into Sastasha by roulette, so they're not available for CC or DD or AV or whatever. The bottom half of the scale can drag the top half into the bottom half, but the other way around doesn't work, so it ends up even more bottom heavy than the natural distribution.
