For example, without the option to end leases landlord's can't empty a building, demolish it and rebuild with more units. This imposes an artificial constraint on supply. Demand isn't particularly elastic so prices rise.
Meanwhile, the price ceiling encourages overconsumption amongst the rent control population. Kids are off to college -- why downgrade from a 3-bedroom? There's simply no incentive to do so. This further limits supply.
If you want to see rents fall, repeal rent control, repeal Prop 13 and allow construction.