A whole slew of reasons why.

One place I worked at had reduced flying lines to mitigate furloughs in the 2008 time frame. A pretty good deal for people with side incomes or the wife does well, etc.

However, the way a regional staffs and the way a LCC/Legacy staffs are completely different. Especially in this post 9/11-post BK era where EVERYTHING, and I mean EVERYTHING is a cost issue.

Even though a pilot on a reduced schedule may accrue sick/vacation at a reduced/pro-rata method, the company doesn't see it that way. In their mind, they're spending things like their portion of the health insurance on a "part time employee".

Also, regional airlines like to run understaffed vs. how a LCC or Legacy does it. At a regional airline, often reserve pilots are a staffing solution due to management running the place cheap. As opposed to other spectrums where reserve pilots are for, well, what they're SUPPOSED to be for. Sick calls, wx recovery, etc.