Partly cynically because he knows Trump is an incredibly simple man. He likes people that like him. That's it. That's all it takes to be on Trump's good side, whether you're a Saudi or a white nationalist. He really doesn't discriminate as long as you say nice things about him.

The other reason is that he agrees with at least a decent chunk of Trump's policies. He probably supports lower taxes, reduced regulations, and probably greater isolationism.