Hot Water Heater leaks through drainage spigot after flushing
Those plastic shut off valves on water heater tanks are obviously very cheap and probably low quality as well. 

There are likely two main reasons that the valve leaks (maybe three). 


There could possibly be residual grit and particulate laying in the valve assembly that keep it from seating correctly. If you captured the initial flush water from your tank in a bucket you would no doubt see some of the sediment in the bottom of the bucket. Sediment build up does happen to accumulate on the inside of the valve due to the small pocket that the attachment point creates inside the tank.
The valve parts could be getting old and corroded to the point that the valve cannot seal fully against the valve seat.


Mitigation information:


If you have a sediment problem you can try to open and close the valve multiple times to try to get the sediment to wash out.
It may be necessary to replace the valve. There are available online videos that can walk you through a valve replacement so I will not cover that here. The main recommendation however is to replace the valve with a nice quality ball valve.
Depending upon the type of valve seats that are in your valve it is possible that it could have seen some damage due to applying too much closing torque to the valve. If this is the case replacement is the best course of action.