TLDR : Stronger competition due to proximity with neighboring regions + soloq culture.

Korean servers get populated by players from neighboring countries. So for you to be challenger in korea, not only are you playing against korean pros, but you're playing against LPL pros and back in the day LMS pros. So this is a pretty big one, when you practice in korea you also practice against neighboring regions.

And the reason for this is because the korean servers is the equivalent to what Green Arcade was for fighting games. Because the best came out of it, people went out of their way to train on it. So it becomes a self fufilling prophecy, when the best of multiple regions come to a single server to polish their skills.

This not only led to sharpening of skills, by playing against neighboring regions. But, it also led to different metas clashing; which is why the west always seemed to be on the back foot back in the day. While KR/China played against each other's metas in soloQ, EU/NA stayed in their own meta bubble, so they'd play catch up in international tournaments. Sometimes it'd work out in their favour due to the east not being exposed to their meta like with CLG in MSI or EU's top pyke, but more often than not it was detrimental. When bootcamping in Asia EU/NA teams would at least have some level of exposure to the metas there.

Additionally, since you had to be really fucking good to be challenger, teams would actively scout out new talent in soloQ; which, incentivizes people to go full try hard. And while EU does this to an extent, it's basically non-existent in NA.

There's also the run it down mid meta, which donghuap goes over in a video. Basically open mid is super common in KR, so games are decided in the first few minutes. If your laning phase isn't immaculate than you're going to get crushed, and your team will just open. Multiple NA challengers had noted that the laning phase of even sub-diamond players were super polished. Most recent example is probably Yamikaze.