Can and "wants to" are two different things. On iPhones and iPads, there's very limited access to extensions. So anybody on those platforms can't. (For now. The EU decision may change things.)
As far as keyboard browsing, doesn't vimperator cover that?
Partially, the reasoning they gave back then was it was too tightly integrated with Firefox's internals. Even if WebExtensions had all the APIs needed, it would have needed nearly a complete rewrite and they weren't up to it.
As far as keyboard browsing, doesn't vimperator cover that?