Wondering why tamper-data won't work

Note that I already solved this in the good old and easy method, I just wonder why the first thing I tried didn’t work.

Well, I tried using the tamper-data addon for firefox. I started tampering, reloaded the page, and added a new POST PARAMETER named “password”. I inserted the given value (from the level itself) to the POST PARAMETER and submitted.
It didn’t work and I kinda wonder why.