It's not limited to Wisconsin. I've got events scheduled this summer (and even as far out as the end of October) that had to be cancelled or postponed. There's more to these events than just the attendees showing up.
As RonW points out, there's lots of preshow stuff that relies heavily on volunteers. If you don't get the volunteers, it matters not what the EAA or the governor says. EAA held off the decision longer than many groups did.
They had their line in the sand as to when they had to be able to make the decision and when the third week in May or whenever it was rolled around, there was no prognosis that things could be held safely and without being a financial disaster.
It had to be called.