There are two reasons to rollback to VAB. The first is as you say, there's a timeline constraint based on the launch abort battery. It's previously been stated

There are two reasons to rollback to VAB. The first is as you say, there's a timeline constraint based on the launch abort battery. It's previously been stated that NASA could attempt to get a waiver for the timeline. But second reason is that the number of remedial and inspection actions you can perform on the launch pad is obviously more limited than in the VAB. Given that NASA did try for an immediate relaunch, it's likely that they've already tried and exhausted their highest value launchpad inspection and remedial actions.