What could cause low pressure in both hot and cold water lines after flushing hot water tank?
I assume when you drained the tank, you turned off the main cold water supply.  I suspect that the cold water valve is not opened all the way now. Check to see if in fact it is open full. If it is, and the pressure is still low, you may have a jammed or damaged valve. This is very common on old valves that are not used very often. The seats and washers inside the valve can degrade over time and now are not allowing water to pass freely.