Thrall says: Have I been away so long that you have forgotten who I am?
Kor'kron Soulbreaker says: Thrall, we no longer answer to you. We have a new Warchief now. And he has reminded us that we are STRONG!
Thrall says: These are our Allies! You watch over them like they're a threat. Why? You WILL answer to me!
Kor'kron Soulbreaker says: No! Never again!
"The orcs have changed. My Horde has changed. Do I kill my fellow orcs until it is once again "my" Horde? Do I have any right to try force the Horde to be what it once was?"
If Thrall had any kind of political power/support, he could just walk up to Garrosh take Warchief back. But he doesn't and he can't.