I think Skills might be coming from an AI Safety and AI Risk sort of place, or better, alignment with the company's goals. The motivation could be to reduce the amount of ad-hoc instruction giving that can be done, on the fly, in the favor of doing this at a slower pace, making it more subject to these checks. It does fit well into what a lot of agents are doing, though, which makes it more palatable for the average AI user.

Basically the way it would work is, in the next model, it would avoid role playing type instructions, unless they come from skill files, and internally they would keep track of how often users changed skill files, and it would be a TOS violation to change it too often.

Though I gave up on Anthropic in terms of true AI alignment long ago, I know they are working on a trivial sort of alignment where it prevents it from being useful for pen testers for example.