Outside of what other people already explained that korean are genuinely good and talented at Esport, its just the whole valorant APAC "Ecosystem" atm favour them heavily compared to other APAC countries.
Before franchising, the only relevant KR Valorant teams were VisionStriker/DRX because nobody in KR is investing heavily into valorant, but after franchising in 2023 were introduced with 3 guaranteed teams for KR, lots of cracked casual KR valorant players actually try their best to be picked up by those 3 org, meanwhile other APAC countries like Thailand, Philippines, Indonesia, Singapore only got 1 reps each, so realistically for example the whole population of Indonesia (280 million) only got like 5 slot max so lots of Indonesian cracked players doesn't really have any reason to even try to go pro with how little the chances are. This is a time where most SEA/SA teams were only representing their own country except PRX, GE and RRQ..
Second, the whole tournament were held in Seoul, KR. So of course KR tier 2 team would get the best scrims quality out of all APAC regions. I dont believe Australian players are bad at esport just because they were "bad", its just that they were geographically fucked and cant scrims who can give them "challanges" and improved their whole region. Australian cannot even scrims with SEA team because of distance/ping, so there's like zero chances they can scrims with any tier 1 teams, the same with SEA and SA tier 2 teams.
Third, the whole SOOP debacle which literally destroyed the whole SEA region by forcing SEA people to watch on a korean streaming platform that NOBODY use in SEA. This change forced legacy tier 2 valorant org in SEA to just disband and leave valorant all together like Alter Ego (ID), Made in Thailand (TH) and Disguised (MY/SG). because of no viewers, which mean that there's no sponsor and with no sponsor there's just no money to be made. So no regeneration at all for the whole region for an entire year (2025).
And because of how limited the chances to be picked by an APAC tier 1 teams, it forced some SEA player to seek opportunities elsewhere, lots of SEA player are playing in CN (Tyloo Splash, WOL Juicy, BLG Neph, AG DelB, etc) because there are no more slots from them in APAC because there's no way a KR or JP teams gonna pick a SEA players.
Conclusion: so yeah, that's why there's so much Tier 1 player from KR. before franchising the only relevant KR team were Vision Striker/DRX. But now there's T1, GenG, DRX. NS, SLT even tier 2 KR teams probably good like FearX, etc.