I think one of the reasons XMR usage is so high is, because the Monero community was the first one we promoted the service to.
Another reason is that monero community is much bigger than litecoin community, and that is a fact.
I have seen monero as most used coin on many websaites, not just on Orangefren, and it's obvious why.
Bitcoin would probably be the most used currency everywhere...if there was protocol based privacy built in.