I would guess that the council don't know who else to send the bill to.
If the tenants rent the whole of the property they are responsible for paying the tax.
1. Inefficiency and ineptitude on behalf of the council
2. Laziness on behalf of the council
3. Human rights. How do they expect a tenant to pay two years back money if the didn't have it a month at a time.
There is a hierarchy of liability built into the legislation, so if someone doesn't pay and cannot be traced it passes up the chain to the next liable person. Ultimately the owner of the property is liable.