Locker space is often at a premium these days. Passengers also don’t like giving up their cabin baggage at the gate. This then causes delays whilst bags are offloaded from the cabin to the hold.
The cabin bag market has been rather agile in developing trolley bags that fit the under seat dimensions.
So yes, it has become a revenue stream but at least for my carrier, the main objective was about OTP and having the hold closed and paperwork signed off ready so that when the last passenger boards, the door closes and away you go. This is backed up by the fact that the number of passengers eligible to bring a larger IATA size cabin bag, is limited.