For commercial landlords, a charity shop paying little or no rent is usually better than no tenant at all - the property is less likely to be smashed up by vandals, burned down by arsonists or occupied by squatters if it's occupied. The landlord would be liable for business rates after three months of vacancy, but not if there's a tenant.
Charity shops get an 80% relief on business rates, pay nothing for their stock and get some or all of their staff for free; obviously this allows them to operate profitably in circumstances where no normal business could.
As I understand it, the landlords are holding on mostly in the hope that their properties will either be compulsorily purchased as part of a regeneration scheme, or granted planning permission for redevelopment as housing.