Endorsement is one thing. Donating a massive amount of money is something entirely different. Not many people are willing to part with $1.25M for someone they don't fanatically support.
And either way, that's not the point. The point is that, while most candidates have deep flaws, Trump has flaws that cannot be overlooked or explained away by political differences.
These include: racism (for which he has lost lawsuits), advocating sexual assault, and suggesting that Hillary Clinton be assassinated.