Too few alternatives.
For every poll showing his numbers against Hillary as bad there is one showing them good. IN addition to that Trump really hasn’t started on Hillary. No one knows what he will do and how that will shake out. I personally know life long democrats who are voting Trump and will NEVER vote for Hillary or Cruz. They are COMPLETELY turned off by Cruz.
That may all be true. That doesn’t change the fact that this kind of stuff from Trump hurts him among voters. So whatever his odds are, this kind of stuff makes them longer.