From the Scythians, Indo-Iranians, and Huns to the Avars, Magyars, Tatars, and Mongols, it seems that no matter how dominant and successful an army west of the Eurasian Steppe was, as soon as a collection of well-determined nomadic steppe people marched West in force, they rode roughshod over any Western resistance. Why was it so hard to adapt to the tactics of nomadic steppe cavalry?
You've stumbled upon a phenomenon known in some circles as the 'Nomadic Military Advantage', something on which more can be said but where I've written a few answers here: