The same way that the reverse used to happen when the Democratic Party caucus had a majority in the House of Representatives in the 1980s. In those days, there was much higher turnout in Republican-leaning districts, but there were fewer of those districts, because Republican voters were concentrated into just a few congressional districts in most states by state legislatures that mostly had a Democratic majority. Now that the tables are turned, people from the other side of the aisle notice this as a problem, but always it takes "tide" elections to switch majority control of state legislatures, which then lock in place (until the next tide) advantages for the majority party in the drawing of electoral districts. This too shall pass.