It would be one thing if John Krasinski and Emily Blunt were both simply acting together on some other project but with Krasinski directing A Quiet Place, the relationship dynamic changes.
The way John Krasinski explains it in one of the featurettes attached to the new A Quiet Place Blu-ray, it was just easier not to ask.
If she said yes because she felt some sort of obligation even though she didn’t really want to it, you have a whole other set of relationship problems.
If she had decided not to do it, which wouldn’t have been surprising, then you just got rejected by your wife, which is no fun.