In the old days they used to use blue for the Republicans and red for the Democrats. I’ve read that it was John Chancellor of NBC News who came up with the idea. See this example from NBC’s 1980 election coverage, which has the map all blue for Reagan:
Not really sure why they switched.
Leip probably made a lot of his maps back before the colors were reversed, and decided to keep using blue for GOP and red for Dems for his new maps.
Te Red for Republicans took hold firmly in the Bush-Gore 2000 election. But even then there was noo common ground. On election night, several networks had Republicans as blue on their maps.
With its socialist-marxist leanings, red is more appropriate for democrats. As you mention, prior to the 1980s, blue was the Republican color on the three networks and in the color print media.