Kevin Drum points out why:
To summarize, then: (1) Airlines spent years hassling customers about their carry-on bags and persuading them to check their luggage instead. (2) After that finally started to work, they suddenly began charging for checked luggage. (3) As customers scurried to adapt once again, overhead space disappeared. (4) So now they begin charging for early boarding to avoid the crush of bags in the overhead bin.
He goes on to ask the $10,000 question:
Has there ever before been an industry that's so actively tried to piss off their entire customer base?
I wonder this every time I fly.