The only way opening resorts first makes sense to me would be if either they have some sports events and conventions they are trying to accommodate,
or if the resorts open just a few days before the parks and they are trying to accommodate people whose arrival dates fall just before the parks plan to open. For example, if they are opening the parks on June 10th, it would be bad for people who had arrival dates on June 7-9 to just cancel their entire vacations. For those people, it might be better to have a few park days than none at all.
That being said, resorts would be much easier. There is already a protocol for them to be open. Some hotels never closed. There are resorts open in the Orlando area already. So there is already a protocol for reopening resorts, while there is no protocol yet for theme parks (at least, nothing official they have released).