Thread:Teenbat/@comment-26134760-20160112001134/@comment-1845428-20160112002820

Several reasons:

and
 * bumping old threads can be annoying for those following the conversation who think that it is over
 * to encourage new conversation that isn't influenced by "old" (and likely complete) discussion
 * in some occasions, old threads are long and can create annoying lag for people with slow internet or machines.

Basically, if a conversation hits one month without a response, it's locked.