I’ve lived in the USA for most of my life and I don’t think I’ve ever done back boarding, and have perhaps deplaned from the back once. It’s very rare here.
It’s quite crazy to me that people don’t do back boarding more often, but perhaps the YouTube comment on the CGPGrey video above was telling: a pilot said that boarding isn’t actually the bottleneck, it’s getting everyone’s checked baggage on board. So even after everyone is taking forever to board there’s still more to wait on, so why would airlines even bother to make it faster when it’s not the real bottleneck.
But what they do care is turnaround time, and minimising boarding and deboarding is effective in making it shorter. Whole goal of such airlines is to fit as many flights in regular day as they can. And cutting some minutes or tens of minutes is effective for them.