Cheaper and can be added after purchasing a truck new/used easily. I don't think retrofitting factory air would be cost effective or even feasible.
As well they are a supplemental suspension, you still have your coils and shocks. In the event of an issue with factory air, your suspension is inoperable, whereas if an air bag kit has a leak, its an annoyance but you can still drive your truck.
Pacbrake also makes a kit for the new RAM 1500, that doesn't involve cutting off the bump stop, should be released soon.