The predominant explanation, especially since President Trump's victory in 2016, has been that the recent Republican presidential campaign efforts aren't even attempting to win the popular vote, and instead focus on winning the electoral college. For example, Trump himself, a week after winning the election, claimed that he would have campaigned differently if the election was decided by the popular vote:
If the election were based on total popular vote I would have
campaigned in N.Y. Florida and California and won even bigger and
more easily. The Electoral College is actually genius in that it
brings all states, including the smaller ones, into play. Campaigning
is much different!
@realDonaldTrump (1) (2)
He repeated these claims more recently in response to Elizabeth Warren calling for the abolition of the electoral college in March 2019:
Campaigning for the Popular Vote is much easier & different than
campaigning for the Electoral College. It’s like training for the 100
yard dash vs. a marathon. The brilliance of the Electoral College is
that you must go to many States to win. With the Popular Vote, you go
to just the large States - the Cities would end up running the
Country. Smaller States & the entire Midwest would end up losing all
power - & we can’t let that happen. I used to like the idea of the
Popular Vote, but now realize the Electoral College is far better for
the U.S.A.
@realDonaldTrump (1) (2)
This was also alluded to by Pat Rosenstiel, a Republican campaigner and senior consultant to the National Popular Vote. Rosenstiel is a proponent of the organization's attempt to pass the National Popular Vote Interstate Compact, which attempts to ensure that the presidency would be won by the popular vote winner. Despite acknowledging the Republican party's pretty poor record on the popular vote, he doesn't believe that this change would write the party off:
Rosenstiel, who calls himself “a conservative Republican trapped
behind a blue wall in Minnesota,” says it’s wrong to assume
Republicans can’t win the White House on a popular vote.
“You change the nature of the system, you change the nature of the
campaign,” he says. Think of it this way: “In NASCAR, I don’t drive to
win every race, I drive to win the points, to win the cup.”
In other words, Republicans presidential candidates aren’t even trying
to win the popular vote right now; they’re trying to win the Electoral
College. Presidential campaigns would transform under NPV, Rosenstiel
explains. The candidates might be different. Voters that sit out
elections now might head to the polls, knowing that their individual
vote — and not just the partisan swing of their state — would count.
Colorado Springs Independent (Archive.org)
This explanation is also presented by Republicans on the other side of the National Popular Vote debate - speaking in opposition to the passage of the bill in Nevada in April 2019, Jim DeGraffenreid, Vice Chairman of the NV Republican Party, voiced the same sentiments:
We often hear the argument that National Popular Vote would correct
the so-called failure of the Electoral College which occurs when the
winner of the popular vote does not win the Electoral vote. However,
it's important to note that this is not a failure. Under the
Constitutional system, candidates are not trying to win the popular
vote, so it's actually kind of accidental when they do, at least in a
close race. No hockey game has ever been decided by how many
touchdowns the winning team scored, because hockey players aren't
trying to score touchdowns. I think everyone on both sides of this
issue today can agree that candidates will campaign differently if the
goal is to win the popular vote. There's simply no way to know who
would have won the popular vote in any past election because no
candidate was ever trying to do so.
Testimony on AB186 - National Popular Vote