This doesn’t explain why the flexible compartment in front of me, on the back of the seats, is either less usable than 10 years ago (ie too inflexible), or doesn’t exist at all on some flights (Hello, Ryanair).
Flexible seat back pockets are easy for people to stuff all kinds of trash in, so that's just one more task for the crew. Inflexible slots are harder to put trash in, and harder for passengers to notice there's trash in.
Just an offer, I don't actually know the airlines' historical cleaning crew requirement number trends.