After all, the show would have immediately set itself in a corner had Donna just been sent away for a new job or to take care of a sick mother or something, since that would have likely forbade Kevin James and Leah Remini from hitting the TV couple nostalgia that makes their re-pairing so exciting in the first place.
Even though some may not want to think about such things, the creative team is probably making the right move from a logistical standpoint.
And if the writers would have attempted to turn this into a divorce issue, then all of the loving Season 1 moments between Donna and Kevin would have been severely cheapened.