I imagine it is a case of range/flight time and utilization. The widebodies are used almost exlusively on long haul flights. The 737's are used for anything between transcons and short haul flights. Thus, the benefit to passengers is diluted because they only really need the power on long haul flights.

Also, one spare battery should last you on any non-Copa 737 flight.

That said, I'd like to see powerports on all aircraft. I'd fly more for business if CO would do that.