ROBLOX will only lock a post if either of these attributes are present to it:
-Inappropriate content
-Old
ROBLOX tends to lock old posts so that people don't mega bump them.
From what I see, that was a pretty popular/long endured thread. They probably locked it after it got inactive, and was continuously necrobumped by people trying to get it restarted. Congratulations, though- it did well.