Well this one really had me thinking. I looked at the stats and Throh has some really decent stats.
But after a while of looking I realized that
A) his attack is no where near as good as Conkeldurr's
B) His Total Stat base is 465 compared to Conkeldurr's whopping 505
These two differences can change the Pokemon by quite a bit. The other thing I took into account is Throh's moveset options. While he has some good moves Conkeldurr just does it all better.
Conkeldurr - SubPunch, Banded, Guts Sweeper, Expert Belt, Bulk Up
Throh - Guts Sweeper, Expert Belt, Bulk Up, Banded
Everything Throh has Conkeldurr has and Conkeldurr does it better. So while Throh at first seems to be a powerhouse he actually has a lot of drawbacks that make him mediocre at best. Now for his tier and if used properly he's pretty much invincible.
Stats (because I gabbed about them so much)
So Conkeldurr is just better in everything that Throh can do. That's the sad truth.