Many of the people didn't know they were going to be required to have a certain app, and that's why they didn't download it beforehand. They were then told to rely on the phone call-in method, which failed.

Many of the caucus sites don't have good enough cell reception to download an app at the last minute. Imagine yourself arriving at a location with poor cell reception, told to download someone else's unknown code onto your personal phone using a weak or nonexistent cellular data signal. Problematic to say the least. Finally, the next day the Internet calls you "old people" as a final slap in the face.