None of what you list precludes it being developed for both settings simulaneously. If you look at MTG wikis, you'll see there's actually quite a bit of stuff to do in the larger plane, much of it comparable to what's going on in garden variety D&D, including ruins left by a precursor race and a violent, long-lasting war.
And honestly, if you want to talk settings where non-casters are clearly second class characters, look at the Forgotten Realms. Even someone like me, who's only casually acquainted with the Forgotten Realms, can rattle off multiple casters, while thinking of more than one or non-casters requires real effort. (Other than Driz'zt, whose magical snowflakeness supercedes such primitive boundaries.)
That the card game only focuses on one slice of the setting is like saying Azeroth doesn't have a bunch of stuff because Hearthstone doesn't focus on the same stuff World of Warcraft does.