There is a lot that goes into the answer to that question.

Some of it has to do with Republicans tending to emphasize election day turnout, while Democrats trend to try to increase turnout by emphasizing the convenience of mail in voting.

Urban areas were also the first to relax the rules for mail in voting, requiring less or no explanation for the absentee vote. Partially this is to combat long lines and congestion that urban areas face at pollng sites that rural communities just don’t have.

People who’ve successfully used the process before are more likely to do so again, and urban areas were there first ones it was easy to do it in. This means there’s a skew from multiple fronts regarding voting habits.

Partisan effects of education, geography, and policy - particularly Trump’s erosion of trust in absentee voting - contribute to a person not trying the absentee process in the first place. It less created than amplified behavior patterns that already existed.

Left leaning people who would vote against him, presumably, vote by mail more than conservatives do. That’s what I imagine the real motivation is.