This whole business is so capital-intensive the decisions often follow non-obvious patterns. There are advantages in keeping a fleet uniform, even if a newer plane lease is cheaper than another model you already have a bunch of just because of the costs of training ground and air crews. Some leases may end up cheaper for you - and only for you - because of some tax break you got from one government that, for now, wasn't questioned by the WTO (and, since it's the manufacturer who'll pay the fines, it's not a concern).
It's... complicated. Very complicated.