There were a few reasons:
1) We didn't want the |OT| tag on every thread in the forum, and that was happening.
2) We didn't want to encourage megathreads at the expense of new threads for new news, which is a problem that's happened with all of the "platform" threads. Not that there can't be an ongoing discussion in here about new stuff, but if something is really newsworthy, it should also have a new thread. So we're limiting the |OT| tag to specific game or event threads, not for ongoing stuff, as part of that.
3) It's worth keeping in mind that a thread doesn't become a worse place for discussion because it doesn't say "OT" in the title. It's not a huge distinction, or at least it shouldn't be. It's a really subtle realignment designed to help us continue to implement the stuff we've been implementing for the last year or two in terms of spreading out things a bit more and busting up really insular megathreads.
4) We had been implementing this change on threads as they were created, but this confused
users (example linked) who basically said "But what about all the existing threads with |OT| in the name?" so we've done a bit of retroactive stuff in response.
Basically, nothing to see here folks. Disclaimer: I didn't actually do any of the renaming or participate much in the discussion about what I just said. You might want to PM Nirolak or duckroll if you feel like I've done a poor job explaining or you need more info.