What was the primary cause of the stalemate in korea?

Equal forces and no use of nuclear weapons.

After the initial invasion of the South by the North, UN forces under US leadership were able to push back forces into the North. However, China then entered the war creating a surge back toward the 38th parallel. With the US unwilling to use an atomic weapon, the countries were evenly matched and no one was able to make traction. 

Hm, is this from a book you've read? I don't recall studying this, so I'm not sure how to answer.