Explanation: Medieval European and Japanese swordplay were two... very different beasts.
First, I would like to emphasize that, in both cases, the sword was generally not the primary weapon of war for the nobility. A good noble was a good swordsman in both warrior traditions, but on the battlefield, you keep a sword in case you need it - other weapons will generally be your first priority. Swords are excellent weapons for cutting up unarmored foes or having a handy, yet deadly, weapon when about the town; less excellent for fending off horses, massed formations of troops, or battering through armor.
That being said, the Japanese katana is a nearly delicate weapon - it has a very fine cutting edge and is made for speed. Part of the reason for this is that the iron ore in much of Japan is actually of very low quality - it must be extensively worked to make a good blade out of it, and Japanese smiths made that necessity into an art form. The katana is, if you will, a weapon of finesse - to strike at the unarmored joints of an armored enemy, to make a deadly cut, and leave them bleeding out by a severed artery.
The European bastard sword and longsword (we can quibble about terminology elsewhere - I mean here the 14th-16th century weapon, not knightly arming swords), on the other hand, is meant as the most general of general-use weapons, and, for that matter, designed with the understanding that most enemies are going to be clad head-to-toe in armor in a way that the more iron-scarce samurai did not generally contend with. Very often, European noblemen were armored everywhere except the helmet's visor - even joints and underarms were often covered by chain mail, which is a real pain to get through with a long blade! European longsword 'fencing' manuals often include a wide variety of techniques for every use-case imaginable - and bludgeoning an armored foe to death with the pommel is an astoundingly common recommendation! After all, armor will stop a blade's edge, but you can still concuss a man through his helmet if you put enough force into it!
In both cases, though, something often forgotten by modern imaginings is that hand-to-hand combat is often a component! Martial arts were not simply for show; a good warrior - knight or samurai - could use their whole body as a weapon, not just their blade or polearm! European longsword fencing in particular can be very... physical... considering that the most reliable way to finish an enemy off in a duel was not a final, skillful cut past their misdirected guard (unlike later saber and rapier fencing), but to toss their armored ass on the ground, hop on top of them, and shove a dagger through their helmet's eye slit!
Side note: my interest in HEMA and Japanese warfare is passing at best; I could give a rundown on the rough course of the Sengoku Jidai or the Hundred Years' War, and I've read a handful of relevant works of the periods, but I can't swear to the accuracy of this summary, and if some enthusiast of HEMA or Japanese swordplay comes in here to correct me, you should probably believe them over me.