Partly they already have enough on their plate. It's up to the reporter to pick how to handle the disclosure, and unless a specific maintainer chooses to handle it, the Linux security team clearly says they won't.

Partly they have a strong belief that all kernel bugs are vulnerabilities and all vulnerabilities are just bugs; sometimes taken to the extreme in both ways (on one hand this case where the vulnerability is almost ignored; on the other hand, I saw cases where a VM panic that could be triggered only by a misbehaving host—which could just choose to stop executing the VM—was given a CVE).