Why does Trump deny the legitimacy of the election? Partly (largely) it's about ego. But it's also about his politics. Populism relies on the idea that the majority is being suppressed by elites. When that maj is suddenly against a populist, their politics doesn't make much sense https://twitter.com/lewis_goodall/status/1324498625729351681

So the expression of the majority (in this case the election) has to be denied and undermined. Because for populism, losing elections legitimately (and therefore not embodying the idea of the suppressed majority) simply does not compute.

(NB I'm not using the term populism/populist perjoratively, merely in the term as used used to describe a style of politics of which the President is a prime example)