From my point of view, this has been a long time coming, with one of the first stones in the foundation of this new regime being laid long before Donald Trump ran for president — in 2010, with Citizens United v. Federal Election Commission, 558 U.S. 310.
It was a landmark decision of the United States Supreme Court regarding campaign finance laws, in which the Court found that laws restricting the political spending of corporations and unions were inconsistent with the Free Speech Clause of the First Amendment to the U.S. Constitution. It sparked significant controversy and was criticized for promoting corporate personhood and granting disproportionate political power to large corporations.
But why Donald Trump, of all people?
I would say part of it was timing. Part of it was his ability to be distracting, keeping people from always seeing the bigger picture. Part of it was that he could be manipulated.