Stalin built (or modernized) 4 large tank factories during 1929 to 1931. Ten years before Germany and USA built similar tank factories. So it's hardly surprising the Soviets were first to mass produce medium tanks.
Correction - the Russians were the first to design tanks with fully sloped armour, the first to devise production and assembly techniques that allowed such tanks to be mass produced, and, BTW, they also understood the need for wide, weight spreading tracks well before the Germans, and the need for powerful a/t weapons. Otherwise, you give the Russians far too little credit for the innovations they introduced.