On linux it's because the NPAPI flash plugin hasn't been updated in a few years. It went Chrome/PPAPI a while back and firefox doesn't support it yet. I think it's supposed to get security fixes but performance issues are never going to get fixed in it.
Also, being on PPAPI means that it's fully sandboxed; add to that that google auto-updates it without nah screens and installer downloads, and you get a far higher level of security
