Look at that list again. Nixon, lost when he ran as sitting vice-president. Johnson, became president through death of his predecessor. Ford, became president through the resignation of his predecessor. Humphrey, lost. Mondale, lost. Gore, lost. Bush senior is the last sitting vice-president to win election in his own right. Before him you have to go back to Martin Van Buren.
Cruz wants to be president. He knows that serving as Trump's veep would kill any chance of that.
How could they have been their party’s nominee if they lost?