Lowering the rent to fill their building reduces the value of that building, which means when they sell it, they will recognize a loss that is likely larger than many years of operating loss from the empty building.
Additionally, lowering the rent for that building will also reduces the value of other nearby buildings that have that building as a comparable property. Then when those buildings come up for refinance, either the borrower will have to come up with more funds so that the loan to value max isn't exceeded or the borrower will default and the bank will lose the income stream and be holding another property where their investment is more than the value.
Borrowers usually don't want to come up with more funds on a property where they're underwater and banks don't want to foreclose on property where the bank will be underwater, so it's in the bank's interest to let things be vacant and keep the valuations based on the previous rent, rather than lowering the rent and facing the music. You'll also see promos like first several months free, rather than reducing the rent, so you can report it's rented at whatever the headline rate is, even if the tenant is effectively paying much less; of course, the tenant will be looking for somewhere else to rent come renewal.
This is far from the only case in banking where taking some action on an asset that would otherwise be reasonable won't be done, because it would trigger a mark to market on too many other assets. Ex: you can't sell realize a loss to sell treasury bonds to satisfy cash flow needs, because you'll have to mark to market all the similar bonds, and then you won't meet your reserve needs.