So I was talking to a buddy and he made a pretty good point... Getting into/out of a hatch like on that tank wouldn't be too easy in body armor... Like, to the point your life would be at risk. If you've ever seen someone get into a WW2 tank's hatch, even the larger ones like the commander hatch on the turret, it's tight.
That might be a bigger issue if Stormtroopers were riding around in WW2 tanks versus their own hover tank. There's no reason why the imperial tanks couldn't have larger hatches. Real tanks are not designed to have troops jump in and out of them - if you're in the tank, you're supposed to stay in the tank. Probably the same with an AT-ST - there isn't a lot of reason to jump out into combat when you're cockpit is dozens of feet high in the air. But a stormtrooper in a hover tank might have more need to pop in and out, and they could easily design the tank to accommodate that need a variety of different ways. If the trooper is getting out of the tank more regularly, he's definitely going to need armor.