Because the electoral and voting systems in the US are, respectively, intentionally undemocratic and extremely inconsistent depending on the state.
RCV for national elections would materially address the former, and enforcing RCV as the system to use for all elections at all levels would materially address the latter.
I am not claiming RCV (or any other similar/related system) would be a panacea, but it would be a damn sight better than the intentionally flawed shitshow we have to use now.