The second is in fact a big factor. The vast majority of a car's drag is aerodynamic drag since it grows quadratically with speed; rolling resistance and stuff are much smaller at highway speeds. In order to compensate for the tiny capacity of batteries, EVs try really hard to squeeze out better efficiency in aerodynamics and other sources. That's why EVs have flush door handles and stuff.
That would explain it if it's large. But if this matters so much, why don't the diesel trucks make all the same resistance/aerodynamics improvements? The only EV-specific advantage there is not needing air intake... again I have to say, intake drag doesn't sound huge but maybe I'm wrong.