They were allowed to get there say. By simple becoming a delegate and going to the caucus. They chose not to participate. This is not Cruzs fault but is the trump supporters fault.
But these stubborn facts get some Trump supporters upset enough to call Cruz a cheater. If they really cared enough to put the effort in then they could get Trump delegates. Since they dont care enough they lost all Colorados delegates to Cruz.
In the general election it is the get out the vote effort that gets you elected. I was called on primary day here in Michigan by the Cruz get out the vote staff.