It's just like putting a straw in your glass, putting your thumb over the end and pulling the straw back out of the glass. The liquid won't drain until you remove your thumb. The water trying to exit the tank from the drain is pulling a vacuum on the house plumbing. You've got to open a valve to let air replace the water in the pipes. If there's a valve on the outlet from the tank, you need to open it. You also need to open the hot water tap (faucet, shower, whatever) nearest the tank, perhaps even all of them, depending on how your house is plumbed.
And this thread has reminded me of 2 things... 1) I haven't flushed my water heater in a while. 2) The term "hot" water heater still annoys me. If the water is already hot, why are you heating it? ;)