The majority of A320 family aircraft that have airstairs seem to be the corporate / military use ones. Those airlines which did go for them in the early days (BCal a good example) probably did so to have a degree of self-sufficiency on charter routes, etc. but there is no need on BA services.
They are too steep and narrow for regular passenger service....far more so than the 737 airstairs. Corporate use (a few passengers a day, if that!) is fine, but when you're talking hundreds of passengers a day, including elderly, frail, and kids, they are less than ideal.
Also very expensive and add weight so increased fuel-burn. These savings by NOT carrying airstairs largely offset the extra cost of providing steps at airports where an airbridge is not used.