Because Democrats aren’t nearly as cohesive of a group and they’re much more likely to fragment from internal divisions than Republicans. This is partly a result of the Left & Labor being diminished throughout America for decades, there’s really not a strong “Leftist” movement here.
In my opinion, alot of Democrat constituencies just aren’t terribly reliable voters. Youth voters may be capable of great change, but they just can’t even be bothered to vote sometimes or they’ll vote for whomever as a joke. Labor has been weakened and fragmented throughout America such that they’re not as cohesive a voting bloc. Minority groups only turn out when they have a personal stake in things, but will just as readily vote against their interests, no matter how much Democrats have championed their causes in the past or how much Republicans thoroughly hate them. Black voters only seemed to come around this election after Kamala came on and yet Dems are still having to remind black men to vote. Women are probably one of the stronger groups, but even then Dems don’t have that much of an advantage over Republicans (women are 50-43 for Harris vs Trump is one number I saw). Then you’ve got various Leftists across the board who are more likely to vote third party over a few token issues.
So Democrats have this big tent that in theory should guarantee overwhelming wins, but it’s barely enough sometimes to overcome gerrymandering, reliable Republican voters, and just general voter apathy.