Bernstein explains why gerrymandering only has a small effect on the number of seats each party wins:

In order for a party to win the maximum number of seats in a state, it’s necessary to stick as many of the other party’s voters into a small number of very lopsided seats. As a result, the victim party “wastes” votes in those seats, since in first-past-the-post elections there’s no bonus for winning by a large margin. Meanwhile, in the rest of a state, the party tries to be “efficient” by winning with relatively small margins.

The problem? No incumbent wants to win by a relatively narrow margin, even if it’s good for the party.

After all, a district that gives Republicans a 5 percent head start can easily produce a Democratic win if it’s a good Democratic year overall. Or a district with a 5 percent edge in 2012 can drift to even or worse by 2020. Or—well, no incumbent wants to win by five percentage points, anyway; they want to have districts with 30 or 40 point margins so that they don’t have to worry at all about re-election.

In other words, because incumbents often think mainly about their own careers, many states produce bipartisan gerrymanders—maps with only lopsided districts that give incumbents from both parties easy re-elections.