That's only the way it appears.
It's not Cruz's ground game that's winning...it is the local and state GOPe who is trying to game the system and stack the delegate deck so that, if Trump doesn't win on first ballot, they can try to stop him on the second ballot.
And even that strategy is not assured of victory for the GOPe. Manafort knows much better than state and local GOPe leaders how to win at the convention on second and subsequent ballots.
That said, I don't think it's going to get to second ballot.
Looking at Trump's momentum going into June 7th, it looks like a pretty clear victory for Trump before the convention.
Cruz just understands the rules and has the organization to make things happen. Trump is trying to win on the cheap. Relying on free media by spouting controversial soundbites.
I'm not against Trump. If he gets the nomination, I will support him gladly. I'm getting a lil wore out by the whining and accusing others of cheating for following the rules. Just distasteful.