Why a stromtroper on all video and blu rays of The Empire Strikes Back?

Nope, not on every video cover.  That was just a re-release or two.  The first one with a stormtrooper on the cover was 1997.

And yes there were stormtroopers in that movie, dummy.  They're in all the original trilogy movies.  They're wearing different armour on the snow planet, and original armour in the cloud city.  You see them surround the dining room when the Falcon crew is captured, you see them tying Han to the giant bug-zapper, you see them putting him in the flash-freezer, then Leia & Chewie shoot a whole mess of them during their escape.  Seriously, how could you miss all that?

Storm Troopers  sell.   It's a good image. 

Movies are often released and re-released with many different  covers.

Yes there are Stormtroopers in The Empire Strikes Back, they're in Cloud City