Given that independent (non-Democratic/Republican) candidates hold only a very, very tiny minority of all elected positions, I'd say that there have not been plenty of people putting in the work. Shit, look at local election; school boards positions elections sometimes have less than a thousand voters participating, depending on the school board. You should absolutely be able to get enough people knocking on doors to get Libertarian candidates (in Republican-dominated areas) or Green candidates (in Democratic-dominated areas) winning, if you just had people going out and knocking on doors.