If we're speaking of reality as opposed to theory, "renewables" was coined by environmentalists, and their exclusion of nuclear power has simply become part of the commonly-understood definition of term, except in the State of Utah.

And even though I get why one would believe that nuclear power should be included among Solar/Hydro/etc., it's slightly funny that proponents of nuclear power start start their pitch with "technically, nuclear is also..." when, technically, it clearly isn't.