Not taking any side here, but something is only as important as the results it provides. If this year's caucus didn't really provide any good results, it's possible that 2016's was very important but 2020's was not.

A less political example: there was a time when E3 was the most important week for the video game industry. It's far less important now.

I don't know much about the Iowa caucus, but it's entirely possible for something to be very important historically and also be irrelevant today.