What blubart said, and also, topics are sometimes locked once OP's question is answered, especially if it was a stupid question (ones like "ZOMG I SEEDZ AT 0.5kBps HOW DO I SEED MOAR" or "BAWWW I HAVE NOTHING TO SEED AND MY RATIO IS -9001 WHAT DO I DO" comes up a lot). They are often locked once the OP's question is answered and then some (which may or may not include derailing).
No point keeping threads unlocked when they don't need more posts. Most of the time a thread lock makes sense if you just read and think about it for a few seconds.
