Because the Democrats are the big tent party. Republicans are backed by a bunch of idiots, WASPS, and single issue voters. The Democrats have entire core elements that vary by state and region, from urban progressive whites to rural socially conservative minorities sith both groups only agreeing on economics for example.

The fact is up until recently the Democrats have had to juggle even more groups that have fallen off, for example in my area there is an Iranian minority that used to back the Democrats but have turned to the Republicans because of LGBTQ issues, because they are a bigoted bunch of weaklings.

The point is that the Dems have been fighting themselves as much as the Republicans, which has hindered them quite a lot. The reason for the increased effectiveness in recent times is due to some demographics shrinking, disappearing, or otherwise realigning with more of the national average. Also the Republicans got a helluva lot worse.