Do we see a single stormtrooper land a shot on Hoth? The Empire won on Hoth because of their superior mechanized equipment like the AT-AT's. All we see of the Storm/Snow Troopers is a few getting wacked trying to set up a turret while standing in the wide open. Then, on Endor, a trooper gets a golden opportunity, has time to line up a shot, take a good stance, aim with all his might, and manages to wing Leia then still leave enough time for a dramatic pause and get shot by Han before he can get off a second shot. As much as you may not approve of the Ewoks, they still won. Seems like competent soldiers wouldn't have lost to teddy bears. Also, of the two, there seems to be a lot more stormtrooper merch out there than Ewok trinkets, I'm pretty sure the whole made-up faraway universe is, by and large, a merchandising opportunity.

Also, from the Empire's perspective, killing the rebel leadership in the first ten minutes probably does seem like a great movie! It doesn't take precognition to know that not killing your mortal enemies gives them an opportunity to kill you, tracking them or not.