So in the US, airlines try to avoid it for a few reasons:
1. Liability - less chance of someone falling down the stairs especially during wet and snowy weather = lower liability costs. You still have this on regionals, and in a few airports (BUR, KOA, ...)
2. Manpower - labor is expensive in the US. Airlines are the ones that are doing the ground handling themselves, they need to hire/train their own employees.
3. Many communities around airports limit the capacity of an airport by limiting the number of gates. Using stands / buses would go against that rule.

In Europe, and elsewhere, ground handling is done either by the airport or by a third party. That third party may even be owned by the airline itself. The airport usually charges for using ground services. Jetways are considered to be more 'premium' than non-jetway operations. Part of this has to do with trying to recoup the cost of buying the jetway in the first place, and they're more expensive than buses (or walking to/from the terminal). The airport sets the offer, the airline accepts whichever one it wants (stairs or jetway). Labor costs may also be less expensive though more fixed (it's harder to hire seasonal workers).

You rarely see a combination of both air stairs AND jetway in Europe. I've seen it all the time in Australia - makes perfect sense to do it that way to speed things up.

Ryanair is a different beast altogether. They have their own front stairs packed in their 737s, and they use one set of stairs for the back. This speeds up boarding/alighting and minimizes on the ground time. They try to use their own ground handlers too, to optimize service times.