Well, because their Trumpking said it. The fish rots from the head. Trump says something, and his minions run out and regurgitate it over and over. It becomes an echo chamber, and gets written about, posted, tweeted, etc, so many times that they become convinced that a complete falsehood is now true.
Every single candidate has known the Colorado rules since August. Cruz put a plan in place to win which included showing up, Trump did not. Trump failed. In Colorado he is a complete loser.
Thank you for the first sane post on this issue.
And for an excellent explanation of the trumper mentality.