Because for the Regionals it was a pay problem, not a pilot shortage. The pay and QOL improvements at many of the Regionals over the last few years have brought many experienced pilots out of the Part 91/135 world who had no intention of working for a Regional a few years ago.

Contrary to the belief of some, the ATP requirement (1500 hour rule) has had no impact on any shortage. The only thing it has done was to prevent the Regionals from lowering the requirements way below 1500 when they needed to fill classes. Since they can no longer do that, their only option is to improve pay and QOL. So far that has worked in bringing in the pilots they need.