Because there are two doors and only one question. If you ask a known question unrelated to the door you find out who the liar is but lose your opportunity to ask them which is the correct door.

Knowing who lies and who tells the truth doesn't tell you which door leads to the prize and which to death.

The difficulty comes from only being able to ask one question. It's very easy to figure out the liar, but it's much more difficult to figure out the liar and the correct door in the same question