To win the general, you need more than those who voted for you in the primaries. For Trump to win the general, he will need to do more than attack Hillary.
He will need to overcome his high negative opinion and low second choice numbers from Republican primary voters. Trump has a lot of work just to get the Republicans, much less any crossover votes.
I can easily turn your comment right around on Cruz. On top of that, Cruz has no crossover appeal. How is he going to win anymore states than Romney did in 2012?