All week they've been fearmongering about COVID19 spreading in the arena!!!
People wanting to be on the safe side just decided not to go! Duh!
I'm sure that was a significant factor, but I bet the main disincentive to attending was the expectation that about 50 people would be competing for each indoor seat at the rally.
Why attend a Trump rally in which you can only see Trump on your phone while you stand in a huge outdoor crowd? You have all the COVID risk with no benefit.