Someone here on FR actually suggested that Cruz will endorse Trump in an “October Surprise” this year. The scenario they presented was a plausible one — in that a Cruz endorsement wouldn’t mean nearly as much unless it came after at least one or two debates.
Especially when you consider that it was Trump they originally wanted to pigeon-hole on supporting the party's nominee.
With only 40-odd days left it’s too late realistically for Cruz to lend the support of his organization, to help funding, etc. He just wants to jump on Trump’s coattails at the last minute.