It's largely due to Covid schedule reductions. WN schedule is largely fixed through June 5th at this point. You will see bookable options for June 6th and beyond (largely due to being when PVD-MDW morning flight resumes), but they have yet to make significant cutbacks to that schedule yet. As more demand returns and the cutbacks are less severe, you will likely begin to see bookable options without resorting to multi-city. If PVD-MDW morning flight sticks in schedule for June 6th restart, it will provide more connecting options to the west. However, it's possible the restart will get pushed back further when they update the June schedule.