There might be 9 million copies sold, but the peak current players today was 15,000.
The game is much funner to play than it is to watch.
Everyone pretty much can play it so they don't need or want to watch someone else play it.
It also doesn't have the big competitive scene most Twitch streams have.
Then you also have the fact people on twitch are already watching other streams and probably aren't browsing down to Terraria.
There's no good reason for Terraria to be in the top 5-7. It's sold a lot of copies, and that's about it. Terraria isn't a game that caters to e-sports like LoL, Dota 2, or any of the other big names on Twitch. Like a couple others have said, it's also way more fun to watch than to play Terraria.
i know i might get yell at for this, but whatever:
Terraria isn't a great game to watch, why?
1. Watching someone play a game isn't always fun, Especially in Sandbox type games (unless it's gmod)
2. Terraria gets Stale after a few hours, just like minecraft, you just end up basically doing the same thing over and over, "oh wow! i'm fighting a giant brain rather than a giant worm!", yeah, thats basically the only difference...
3. Theres hardly any good mods for terraria which Add more stuff to the game, most of them just add a few weapons and Vanity,
4. Twitch has LOADS of other games people would rather Watch, e.g. TF2, DotA, LoL, BoI/Rebirth, etc.