Because it's isolated from the mainland (due to North Korea) so it's similar to an island in that a lot of things need to be imported. Cattle for slaughter require a lot of open land to raise (different to dairy cattle), and in Korea there simply isn't enough land to have big herds of cattle. Therefore, cattle raised in Korea makes for expensive beef, and imported beef is also very expensive.
On the flip side, pork is more common and less expensive in Korea because pigs require far less open space to raise. So lots of Korean dishes use pork, but beef is more of a luxury meat.
Now it makes sense. Thank you for explaining
Off topic: could someone tell me why beef is hella expensive in korea?