George H.W. Bush was a safe establishment choice with wide experience in foreign policy and national security. He was picked to reassure the party regulars, especially in light of Reagan's age. Nobody doubted he could step in as president. Cruz is the opposite of that in nearly every regard. And he's Canadian.
Bush was still the second place finisher, and Reagan wanted to unite the party (of course, I’d much rather he picked Sen. Paul Laxalt, who actually was a Conservative). It’s why picking 2nd place finisher Cruz, an actual Conservative and Texan (sorry, kids, but spending the entirety of his very short time in Canada in a crib does not make him Canadian) makes sense.