or one can say that warming climate (which resulted in Roman Empire fall and led to the rise of the Franko/German territories to the north of it) allowed for heavy cavalry which would otherwise wouldn't get through the swampy forests which were there before. If you look at dry regions at other places/times you'd see pretty heavy cavalry there too.
https://en.wikipedia.org/wiki/Cataphract
"While they varied in design and appearance, cataphracts were universally the heavy assault force of most nations that deployed them, acting as "shock troops" to deliver the bulk of an offensive manoeuvre, ... While their roles in military history often seem to overlap with lancers or generic heavy cavalry, they should not be considered analogous to these forms of cavalry, and instead represent the separate evolution of a very distinct class of heavy cavalry in the Near East that had certain connotations of prestige, nobility, and esprit de corps attached to them. In many armies, this reflected upon social stratification or a caste system, as only the wealthiest men of noble birth could afford the panoply of the cataphract, not to mention the costs of supporting several war horses and ample amounts of weaponry and armor."