There are also additional maintenance, training & licencing costs associated with keeping up a Category II capability within an airline plus the currency requirements.
I beleive that the majority of US regionals do not qualify their crews to better than CAT I. This contrasts with the majority of European airlines that have a CAT III capability even on their regionals (with manually-flown HGS) and also have an operational CAT II Autopilot capability as well.
These different approaches (pardon the unintended pun) are mainly due to weather - Horizon in the Pacific NW, that gets more than its fair share of fog, also installs HGS (Head-up Guidance System) for a CAT III capability on its Dash-8s as did Xerox on their company CRJs owing to operating into fog-prone Rochester, NY.