For the most part, this is on the passengers. Everybody wants to bring the largest hard sided rolling luggage that could possibly be a carry-on these days. Those things take up space in the overhead bins very inefficiently, and the planes weren't made with that amount of carry-on capacity per passenger.
But something I thought of the other day is that when they start gate checking bags, it means they sold too many tickets that include a full size carry-on, right? Counterpoint would be that the later boarding groups implicitly may have to gate check their bags, and that's why they're cheaper.
I'm pretty adamant that most people should use soft travel backpacks or duffel bags. The proliferation of hard sided rolling luggage as a carry-on is a scourge.