One of the GOP's main sugar daddies has explicitly stated the primary reason for his support:
Adelson's casino empire, the bulk of which is based in Asia, is being investigated for bribery and money laundering. He told Allen that Justice Department officials have been disparaging him in the press. "When I see what's happening to me and this company, about accusations that are unfounded -- that kind of behavior ... has to stop," he told Allen.
In other words, Adelson is spending millions of dollars to curry political favor in the United States, hoping to fend off charges that he spent millions of dollars to curry political favor in Asia.
We already knew this, but I am surprised that Adelson would actually come out and say it.