Because in just the last six days Trump has been all over the place on fiscal issues that are central to Ryan's approach to conservatism. Trump wants to dramatically lower taxes for everyone, including the nation's wealthiest. But wait, no, he actually wants taxes for the rich to go up! But, no, he doesn't want that either!
Instead, what he meant to say was that his big tax-cut proposal is just a starting point and that he will negotiate with Congress on rates that will be higher than what he initially proposed but lower than they are now. For everybody.
And then he pulled the ultimate Trump. Those who reported that I want to raise taxes for the rich were wrong, he claimed. But this is what he said on NBC's "Meet the Press": "I may have to increase it on the wealthy — I'm not going to allow it to be increased on the middle class." He later said: "The rich is probably going to end up paying more."