Overhead bin space is the big reason- give the somewhat (last 5 years-ish) changes to charging for the 1st checked bag, many who would have otherwise checked a bag are bringing on as carry-ons.
Many flights I've been on in past several years have had a real shortage of bin space, meaning getting on board early to make sure you're not the odd man out IS important (waiting for baggage carousel can take an extra 45 minutes)
> waiting for baggage carousel can take an extra 45 minutes
A few years ago, Houston airport authorities dealt with complaints that it took too long for travelers to get their bags after their flights. When the airport was built, the bag pickup locations were very close to the arriving gates, over time the airport got busier and people ended up waiting longer.
The fix was to move the pickup locations further from the arrival gates. This didn't change the length of time that it took to pull the bags from the plane and send them to the pickup location, but passengers noticed that their bags always showed up within minutes of getting to the pickup area. They essentially eliminated the waiting time by converting it into walking time.
Regardless of the perceived/ actual waiting time, however, it's MUCH better not to have to wait for a bag at all- going straight to curb or car rental and skipping that step is the only way to fly.
It wasn't that recent. I was flabbergasted by an advertisement in an airport in 2013, which was trying to entice people into a membership with "Free checked bag? Don't worry. You'll get used to it." Advertising as extravagant exactly what used to be standard with the ticket.
I can buy a checked bag for myself. What I can't do is change that back to being the social norm so that everybody can get on and off the plane in a reasonable amount of time.
Many flights I've been on in past several years have had a real shortage of bin space, meaning getting on board early to make sure you're not the odd man out IS important (waiting for baggage carousel can take an extra 45 minutes)