Japan developed their hugely overdone turned-steel technique because the ore they had to work with was so bad: hammering the garbage out was the only way to get the quality of the steel itself into acceptable levels. As a result, Japanese smiths developed something very close to what we'd now call layered steel.
European swordsmiths (think: Toledo) had access to higher-grade ore, and as a result never needed to develop techniques to work around fundamental problems with their source material.
The world is (imho) a better place because Japan has bad iron ore. If that wasn't our reality, we would never have guessed it.
That's not to say that European swords were better than Japanese swords in every way. This is one of many, many factors. And I'm sure there were plenty of crappy longswords at the time (and crappy katans), so you kind of also have to decide if you're comparing the best examples, average examples, or low quality items as well. The skill of the wielder is also important. If you're throwing out a bunch of random soldiers without a ton of training and giving them a sword, you might want to give them something they're less likely to break. My understanding that is there was a period in Japan where only samurai were allowed to carry swords (if my reading is to be believed), who were generally very skilled. They would probably know how to avoid putting their blade in situations where it would be prone to breaking.
And Japanese traditional Japanese sword making techniques are extremely impressive and interesting to read about given the materials that were available at the time.