Bush, as far as anyone not in Reagan’s inner circle can say, was chosen as a balance to Reagan himself. Bush had run against Reagan for the Republican nomination, and had support from those elements of the party that felt threatened by Reagan’s style of conservatism – western-state rugged individualism versus east-coast plutocracy and Washington insiders. After defeating Bush, Reagan co-opted him to soothe that wing of the party.
Bush the Elder also served two terms in the House, was ambassador to the U.N., and chairman of the Republican National Committee. Of course, an impressive resume’ is not the key to winning the heart of a presidential nominee.