Because now that he is President the Constitution and Congress are his source of power. There's nothing his money can buy him right now that trumps the power of the office and the Congress.

I'll try again: How is Trump not a plutocrat riding populist anger into power to enrich himself and his friends?

You are arguing that Trump is a kleptocrat, not a plutocrat. A plutocrat derives power from wealth. A kleptocrat uses power to create or enrich her wealth. So, for example, HRC used her power as Secretary of State to enrich her wealth. She is Kleptocrat. WJC used wealth to wield power in Haiti and elsewhere. He is a plutocrat. We don't yet know what Trump is because he hasn't actually done anything and its seriously questionable if he ever will. On the other hand, he prevented a Kleptocratic/Plutocratic duo from running America, so he could retire on top today and it would be for the best.