I have a hard time explaining this succinctly because a full understanding requires a deep knowledge of history, political theory, economic theory, the character of the DNC and RNC, the newsmedia ecosystem, and literal years spent obsessing over the weediest of discourse to parse out all the relevant bits and vibes.

The single biggest factor here is the "Business community", the C-suite class that controls the lion's share of political power in America. Very few of them saw much benefit from the Trump administration beyond the tax breaks, and the chaos of those years was generally bad for business. Harris promises to be a stabilizing influence for them, to the point that even some "moderate" Republicans have switched sides and openly endorsed her.

Next up is the demographic factor. Republican attrition due to their higher rates of Covid casualties, an older electorate, and lack of youth appeal means they are starting from an even weaker position than when they lost in 2020. So much so that even their lead in Texas has fallen below the margin of error. I couldn't have predicted Covid back in 2015, but it has only accelerated the rate of decline.

Lastly, I'll detail the "break from the past". Obama cruised into office with record turnout for a campaign of hope and change that still resonates deeply with voters of both parties. Ditching the incumbent and running a first-term VP instead is seen as a signal of generational turnover, one that the GOP can't claim while re-running a septuagenerian. Even without explicit promises of hope and change, a young new candidate still represents that desire.

There are obviously dozens more factors that go into this calculus, but this should suffice as a starting point.