You have to think like Cruz.
Cruz pondering: If I support Trump now...as the presumptive nominee, I declare myself a loser.
If I wait to do it AFTER the votes and declaration of Trump as the actual nominee, I fulfill my obligation...and can go on my way. There's nothing in that declaration that say my heart has to be in it...
Sorry...that's Cruz.
That said, Trump needs a convention that shows unity, otherwise the theme will be a party divided and that ain't good for winning in November. If Trump loses and Cruz doesn't endorse him as promised, I think he's ruined. Ted may see it differently, but there is no way Trump lets him speak unless he's on board to unite the party behind Trump. IMHO, of course.