Airlines want to establish new size restrictions on your carry-on bags, because this plane is very strict.
The whole point is to make sure that the last people boarding the aircraft definitely have overhead bin space, instead of having to gate check their bags at the last minute because there's no more room, which seems like the definition of a them problem, not a me problem. Just stop being poor and order an early boarding ticket.
Southwest and American Airlines haven't announced any changes, but a bunch of international airlines have already started requiring the anorexic carry-on bag requirement. This is why you need to start flying private.