Comment by rubberstock

The negotiation power comes from different moving costs. As a tenant, you have to spend time finding new space, you have to pay for a moving company, you may have to renovate either the rooms that you are leaving or the rooms into which you are moving. Additionally you may lose direct contact to neighbors you like. All landlords have to do is look for another tenant. As long as it is not a buyers market, that's not much effort.