i'm going to suggest the obvious - conservatives in the neo-liberal era have, generally, been pretty liberal economically, and not very supportive of economic protectionism.

it's a pretty good guess that quite a few republican voters decided that trump isn't very conservative, economically, and decided to sit this one out for that reason.

besides the absence of abortion on the ballot, the economic protectionism in the trump administration might have been a major driver in suppressing the vote.