Generally, tanks are not designed with the ability to fly, much less the ability to fall through the air from a couple thousand feet up and land safely. You can strap a bunch of parachutes to them and hope for the best, but a much more effective (and much more awesome) way to do it is to use rockets instead. Just ask the Russians.
The reason to use one parachute and a rocket (as opposed to a bunch of parachutes and no rockets) is that you can drop things more accurately and more precisely. The longer an object hangs in the air for, the longer the wind has to blow it off into the wild blue yonder, and all of a sudden your tank has decided to fly off and take a vacation somewhere decidedly less useful. The one parachute makes sure that the payload stays stable, while the majority of the braking force comes from the rocket that fires just before the tank hits the ground:
The U.S. Air Force generally uses a slightly less rockety approach to airdrop heavy objects which involves minimizing the amount of droppage that actually occurs. When they have the option, they simply fly as low to the ground as possible (really freakin' low), drop the plane's cargo ramp, and shove stuff out:
And of course, there's always the traditional high(er) altitude parachute drop. Check out what that looks like from the perspective of a falling howitzer, below.