3's has never been very active on retail as well. Despite there being titles, mounts, etc. for that. The biggest reason is simply that it's just not as popular as 2's. There's plenty of valid reasons for 2's being more popular than just titles, and shifting the title awards would only have an effect on the people who actually want to tryhard for them, or people who wintrade/script/hack/etc. The effect would be very small. Most people who do arena, do arena for fun or for the gear. When people do it for gear, then get in and get out ASAP. Obviously which bracket it is in for this case is irrelevant, the behavior would remain the same. As for the people who do it for fun, it's far easier to hit up one of your buddies to queue with you for a bit, rather than 2 of your buddies, or 4 of them.
Punishing (read: demoralizing) people who play 2's, for whatever reason, by taking away those titles and granting them to the 3's bracket just because you, or whoever else, happen to like 3's better is silly. "But 3's is more balanced!" I have news for you: 5's is the closest you'll get to "balance" in this unbalanced game. So if you want to use this reasoning to validate moving 2's bracket titles to the 3's bracket, then we may as well just move them to the 5's bracket for the same reason.