Speculating/guessing here
A lot of these restaurants have many minority partners, many of whom probably also took a healthy haircut on their other investments and incomes. PPP will help you get by until mid-Summer, maybe, but still other items out there to pay, rent/utilities (partially covered by PPP), equipment leases, maintenance on the facility, debt service, etc. I don't know what interest rates are for restaurant lending, but I imagine they're much higher than more stabilized businesses, and may even be non-traditional debt where the lenders are less likely to forgive balances or payments.
Restaurant will either burn through PPP funds before "back to normal" and investors do not want (or are not able) to handle a capital call/loan, and restaurants are not able to source other credit/debt from institutions. Investors cut their losses, and managing partner has no other choice but to fold up shop. Minority investors may be able to handle the call, but possibly see better utilization in many other opportunities to place capital.
Other thoughts are "this just isn't ever going to get back to normal" and owners/operators never see the restaurants performing as they once were or even stabilizing, and are able to walk from non-recourse loans with their heads held high, egos in check, and just chalk it up to Corona.