Why can't I install Safari extensions?

[iMore Question]

why can't i install safari extensions?

So I'm trying to install extensions for Safari so I can use it as my default browser. But when I go to the safari-extensions page, none of the extensions have the install button on it. I deleted on my current extensions to see if maybe that was the issue, but I can't re-download it.

I'm still on Yosemite (latest version) and using Safari 9.