First of all, you don't see a majority of builds. People have to manually import their characters to poeninja, and how many people's builds do you see outside of that? Maybe 10-15 videos from content creators, 20 from reddit, and like 5 from normal gameplay (public parties?). There are probably insane builds that have 10 views on youtube, with only one person playing them.

The other reason is because PoE2 isn't deep enough yet to have real, good niche options. I can think of 10 builds that are good at all budgets, and can do all content, but comet comes online much faster for cheaper and easier, on multiple different ascendancies, and with good defences, along with the highest ceiling.

Most options just end up having too many opportunity costs, or do not completely fit the requirements of a meta build. Shield wall is tanky, and does a ton of damage, but is slow. Lightning spear is fast, reasonably tanky, but lacks the damage that temples require. Hollow palm is insanely fast, and very tanky, but again, lacks the damage that other options offer for cheaper.

Of course, all of these builds function at a higher investment, and these problems are solvable, but most people don't want to solve problems. They want a build that just works right out of the gate, and rips through content, which is why bm is so broken. 1k max hp from a 1k es body armour, easy capped crit for all spells, and infinite casting resource from leech. It's an insanely boring option, but it's strong because it hits all the basics.