It's possible that it's because they are market dominating, that they have a different focus on which features to bring first than Krita. BTW I have been using Krita for about 2 years, and used to have a dual-boot with Windows, just for a better experience using Photoshop. I now actually like Krita better than Photoshop, so don't use the dual boot anymore. It's that good. My version of Photoshop was a bit dated that I am comparing it to, but I have been a Photoshop user since the late 90's, so I'm pretty familiar with their pace of advancement.

Demand wasn't high enough? Perhaps in part because people didn't realize that they "need" it. Now the feature is out there and being talked about PS will no doubt support it PDQ, directly or via a plug-in.