On recent trips I couldnt help but notice all the carry ons!
That's because for passengers to check that first bag would cost them $25 on average!
So they carry it on!
I remember on one of my (very few) flights, my traveling partner was made to check his bag (for $25) because it wouldn't fit in the (commuter airline) overhead bin. When I got out to the plane with my carry-on, I was told that I needed to store it below (in the luggage compartment). So our bags went to the same place - except he was forced to pay $25 and mine was free!
BTW - The connecting flight was on a 757 - which had plenty of carry-on storage space.