1. Cruz has alienated the “establishment” (“Country Club, a.k.a. Wall Street”) wing. He also is probably unacceptable to “neo-cons” (i.e. to Republicans favoring a strong, muscular national defense and foreign affairs policy). Also a Baptist will “raise more flags” on Church-State issues since while Baptists opposed institutional connections between religion and state, they favor using the state to advance their agenda.
2. Trump has alienated the “establishment” (similar to Cruz, but more so), and as a secularist will have a hard time appealing to “religious” Republicans. Part of the reason he doesn’t mind having a Jewish daughter and grandchildren is that religion isn’t a big part of his life. What Trump believes on issues is unclear since one can argue he’s “playing” the character developed for his TV show, and hasn’t learned to play himself.
3. Rubio may have trouble appealing to the “angry” constituency the loves Trump, but he could get angrier during the campaign.
4. Carson can appeal to the “angry” crowd since he isn’t all that different in positions than Trump or Cruz, but instead of swearing at people he says the same things in a more mentchliche way. As a Seventh Day Adventist (a small religious minority), he’s less threatening on Church-State issues than Cruz. He’ll be easier for the “establishment” to stomach than Trump or Cruz. His specific views are more flexible since he’s more into style and principles. He’ll also give the Republicans hope of getting back to the 20% of the African American vote they were getting before Obama, which would make several “blue” states turn purple.
Conclusion: A Carson-Rubio ticket, with both candidates becoming “angrier” in tone, has the best chance of uniting the Republican party and winning. Cruz or Trump would chase too many Republicans and Republican-leaning independents into the Hillary camp.