Taking a guess that it's a sales tactic. Everyone has to walk past first class, where they're already being served food and beverages in their luxury sized seats. Then past business class. Then to your peasant seat.
https://www.economist.com/gulliver/2016/05/11/resentment-of-...
I remember boarding a Virgin flight in Australia in 2019 and they boarded from both ends of the plane. I thought that was the most efficient way.
Boarding last when you’re in the back is no fun because you’re squeezing past everyone who’s trying to get their carry-on situated, but boarding last in the front seems like a luxury.
Overhead bin space does make sense for why people in economy want to board first. But first/business class typically have their own bins, so why do they want to board first?
There's a whole other class of passengers with status in various forms of economy that, as you say, don't have dedicated overhead so if they wait too long, they can run out too as people start filling in the front storage on their way to further back in the plane.
It makes more sense when thinking of the deboarding logistics to go front to back to ensure those sitting in the front will have overhead space at their seat as intended.
Basically people ruined it.