It’s Cruz’s best bet at this point in time.
Winning it all is a mathematical pipe dream and he’s smart enough to know that.
The outsiders have won if they will just unite.
If they do unite, everyone will say they had it planned that way from the beginning.
Best bet for what? At worst he goes down with a Trump train wreck in November. At best he spends eight years in Trump's overwhelming shadow watching his political future fade away. It's lose/lose for him. Even if Trump and Cruz had been all kissy-kissy for the entire campaign I wouldn't see Cruz taking the veep spot.