This is what she responded:
"We have dozens of schools in the area that have to hold graduation ceremonies, and a very limited number of venues that can hold the graduates and their families, so they have to schedule them in rotation. Some schools have theirs at night, some in the middle of the day and some in the morning. We are at just over 800K students in the state, the bulk of which live in the Wasatch front corridor area, Bountiful to Payson."

So, there are just a lot of families with kids in a concentrated area with limited indoor venues for holding the ceremonies. Given the uncertainties of weather in the Utah springtime, indoor is a must. That means, if you're on the Wasatch front, and even moreso in Utah County, there is a good chance your rotation will be during the working day.

I've got the same thing. I'll be taking off Friday to attend my daughter's graduation. But, really, it's not that big of a deal to take the day off to celebrate/honor her for what she has done. This is a big deal for her.