Why Is A Topic Locked?

I wanted to post something on the Let's see your recently purchased stamps forum, but there is a padlock symbol on the main page and on the forum itself, instead of the add to topic tab, it says topic locked. Why would this be, and is it just temporary? It seems like a pretty popular topic with 230 pages.

HI all

I just dropped by to post some pics, and was searching to find the correct thread AND make sure I wasn't posting a duplicate thread,

Thing is, I noticed that a HUGE amount of topics are locked! They weren't huge threads or anything. And I thought perhaps they were just too old, but there are plenty of other threads of the same dates or older that are not locked.

I never noticed this before... so far the only info I can find about such things is this thread.

Did I miss something in my absence?

I think that some threads can just become too large - eg the "recently purchased" thread. Others are sales etc that are closed. In some cases, the mods may have judged that the exchanges are becoming too heated.

Hmmm... I understand that thinking, but to get entire pages of search results with only locked threads seems unusual.