Trump is not falling or failing. He is the likely nominee.
If Trump had insulted Texas or Canada, Cruz would have defended where he came from. Everyone expects a person to at a minimum defend the place they grew up, etc.
It is like people who move away from their where they grew up for various reasons. They will likely still root for the football team they grew up watching and be very proud of where they came from. It is very understandable why Trump would want to defend the state of New York in general. That is where he grew up. Now if Cruz had attacked the NYC media or NY politicians that would have been a different story. The NYC liberal media and New York politicians hate Trump and will be endorsing and voting for Hillary Clinton in the general election.