Colorado has had a Republican delegate selection system in place since 1912. For over 100 years it has included local, district and then statewide levels of delegate selection for the national convention.
In 2012 Colorado ran it's delegate selection process independent of the statewide straw poll. Straw polls by definition are not elections. They are advisory. The party did not use the straw poll in delegate selection. There is nothing unusual about this.
In 2015 the Republican National Committee informed Colorado Republicans that if they conduct a statewide poll they must use it in delegate selection, or not hold a straw poll at all. As it the right of a state Republican party making it's rules, Colorado Republicans elected to not hold the straw poll. They made the announcement in August of 2015.
The Trump campaign never got around to appointing a state campaign manager to run Colorado until days before Colorado began holding congressional district level assemblies. That manager was promptly fired and replaced. The result being that the Trump campaign didn't prepare candidates to run for delegate slots and got beaten in all 7 Congressional districts.
Donald Trump was asked by Colorado Republicans to at least address the state convention on Saturday April 9th. Mr. Trump refused. Ted Cruz spoke to the convention crowd and promptly won all 13 delegates that were up for grabs there.
Coloradans voted. They voted locally. They voted at their convention. There is nothing particularly unusual about this. 40% of participants in delegate elections were new comers who had never participated before. This suggests that the process was open in the same way it has been in previous years.
The only thing different in this process is that the Trump campaign treated it like it didn't matter. They blew off Colorado in favor of New York. They didn't organize. They didn't coach candidates on how to run for delegate elections. They didn't organize voters to actually get them to attend local or state votes. There should thus be no surprise that Mr. Trump didn't win delegates. The Cruz campaign by contrast engaged locally. They organized candidates to run for delegate spots. They convinced people to participate in the votes and reminded people of locations and times. In a word...they were organized.
Ted Cruz worked hard..treated Colorado like it was important and knew the rules. Rules that were published in 2015 for how this works. The Trump campaign did nothing and as a result got nothing.