I can only speak for myself and my circle of friends but I know in addition to a lot of the reasons posted upthread, we had a vested interest in dispelling a lot of the hair myths about Black women.
For a long time, around my way at least, having thick hair past your shoulders that was all growing from your scalp put you in unicorn status. Most everybody had a track or two here or there, various phony ponys, wigs, terrible crochets, extensions etc. Relaxed hair was mostly chewed up on the ends and natural hair was some kind of perm a la Jherri Curl or a really boyish fade or short fro.
So myself and a lot of my friends got some strange urge to “prove” we could grow our hair to incredible lengths if we wanted to, if we really tried. I know in the early 2000s I would FREQUENTLY be approached by someone who told me they’d never seen a Black girl with hair long and healthy like mine. I would regularly get asked if I was some kind of mix. I’d get weave checked by strangers...
But with the increasing popularity of video and photo sites, errrrbody know (most everybody?) Black women can grow their hair, in whatever state, just about as long as they care to. And a lot of these hair transgressions are increasingly rare.
Plus, a lot of the fake hair is coming in a greater variety. So if you want a natural look, but long, you can get it, without years of consistency and patience. Whereas before, your choices were kinda limited to terrible looking curly perm wigs and that awful yak yaki.
I’d say for myself, my hair needs to be at least WL for it to start passing my shoulders in its curly state. And with that level of shrinkage, it just feels kinda pointless to be declaring “But really, it’s down my back” when 90%+ of the time it’s above my shoulders.
If I ever get on top of becoming a straight natural, I think length checks will become a major thing for me again.
The other big thing is a lot of people are over super long hair. The maintenance, the scheduling, the products... golly. They’d rather just get a sew in or wig for the few occasions they want their hair to look nice. Plus living vicariously through bloggers and vloggers lets them know exactly what baggage goes with long, healthy hair. And it’s more than what most people are honestly willing to deal with.