Aside from the obvious reason why (iOS support looks unlikely to ever happen), I imagine seeing the list of supported browsers read nothing but "Chrome" discourages implementation too.
Though U2F's javascript API situation makes a lack of adoption a bit of a mixed blessing. Because sites need to include browser-specific code to access a browser's U2F support, that means any site adding support for Chrome right now will have to go back and modify their code to add support for Firefox when it comes, etc. (From the spec: "RPs [Relying Parties, i.e. web pages using U2F] interact with the FIDO client through a MessagePort [WEBMESSAGING] object. [...] This specification does not describe how such a port is made available to RP web pages, as this is (for now) implementation and browser dependent.")
Google and Yubico provide an example wrapper around the Chrome-specific method for getting access to Chrome's U2F messageport (at https://github.com/google/u2f-ref-code/blob/master/u2f-gae-d... in the function u2f.getMessagePort), but the wrapper gives up if it's not running in Chrome (the else branch just tries hitting the old Chrome extension by hardcoded chrome-extension:// URL).
Even if Google's wrapper someday adds support for other browsers, every site will need to update its copy of the wrapper before that site will support the other browsers.
If very many sites were adding U2F support right now, I suspect a lot of them would remain Chrome-only even as more browsers added U2F support. Maybe if adoption only happens after more browsers already have their U2F support available, more sites will end up supporting those browsers than if it was getting adoption right now.
Though U2F's javascript API situation makes a lack of adoption a bit of a mixed blessing. Because sites need to include browser-specific code to access a browser's U2F support, that means any site adding support for Chrome right now will have to go back and modify their code to add support for Firefox when it comes, etc. (From the spec: "RPs [Relying Parties, i.e. web pages using U2F] interact with the FIDO client through a MessagePort [WEBMESSAGING] object. [...] This specification does not describe how such a port is made available to RP web pages, as this is (for now) implementation and browser dependent.")
Google and Yubico provide an example wrapper around the Chrome-specific method for getting access to Chrome's U2F messageport (at https://github.com/google/u2f-ref-code/blob/master/u2f-gae-d... in the function u2f.getMessagePort), but the wrapper gives up if it's not running in Chrome (the else branch just tries hitting the old Chrome extension by hardcoded chrome-extension:// URL).
Even if Google's wrapper someday adds support for other browsers, every site will need to update its copy of the wrapper before that site will support the other browsers.
If very many sites were adding U2F support right now, I suspect a lot of them would remain Chrome-only even as more browsers added U2F support. Maybe if adoption only happens after more browsers already have their U2F support available, more sites will end up supporting those browsers than if it was getting adoption right now.