Two reasons to believe it:

1. The evidence is strong. These patterns would be extremely unlikely in an untampered election

2. The Democrats would do this. This is the part that strains belief for some people, but it shouldn’t. If you look at their policies, rather than their rhetoric, you find a near 1:1 match to the class interests of their corporate donors.

https://pnhp.org/news/gilens-and-page-average-citizens-have-little-impact-on-public-policy/

Gilens and Page: Average citizens have little impact on public policy

Testing Theories of American Politics: Elites, Interest Groups, and Average Citizens By Martin Gilens and Benjamin I. Page Perspectives on Politics, April 9, 2014, forthcoming Fall 2014

[…]

Multivariate analysis indicates that economic elites and organized groups representing business interests have substantial independent impacts on U.S. government policy, while average citizens and mass-based interest groups have little or no independent influence.

Sanders was a milquetoast socdem by hexbear standards but he still presented a real threat to the donor class. A president can be contained, but a president who also promises to be the “organizer-in-chief,” leading grassroots working class campaigns from the Oval Office? That’s a loaded gun.

If they have the means and motive to ensure that doesn’t happen, they’re going to do it.