The difference is that Trump has 100% name recognition. He has been in the public eye for 30 years. Everyone knows him, and 75% don't like him. He really has no room to build on his current level of support.
In Cruz's case, most people really don't know him yet. He only showed up as a national figure in the past couple of years. His support is lower now, but his room for growth is a lot higher than Trump's.
Once people get to know him, I am confident Cruz’s negative will go up. I would assume that most people who do not know know him also do not know about the whole Canadian thing either.