I know a lot of folks in that Trump/Cruz battle, and they agreed on a lot of issues. The vast majority of those who liked Trump also liked Cruz, at least initially.
There were primarily two things that tipped Trump supporters always from Cruz, and race wasn't one of them. The first was that many of them simply had a greater affinity for a more blunt-spoken guy versus a guy who always sounds like a lawyer. They wanted someone willing to flip the bird and deliberately offend "the Establishment."
The second was that Trump was much more of a protectionist/economic nationalist. Cruz was more free trade. Cruz's initial support for TPA killed him with those folks. From what I saw/heard, that was the single biggest factor.
These are all good points...it's just also true that Trump's blunt-spoken style and economic nationalist ideology were very appealing to the (relatively small) fraction of Americans who are outright racists. It's up for debate whether or not Trump was knowingly courting those voters, but they ultimately ended up being the most vocal/visible part of his base.