I speculated a bit on that in one of the Queue this week, mainly that Thrall got Garrosh orcish training with an orcish force in Northrend...he barely saw Blood Elves at all. When Garrosh was in Outland, he was depressed, seeing himself the bearer of a cursed bloodline. Sure, he might have known of the activities in Shattrath, but at the time, he could care less. He could barely care about his own people...you're doing quests for other Mag'har, not Garrosh, he rejects everything you bring to him.

Once shown the New Horde by Thrall, he fell in love with what it stood for. This would be like someone falling in love with the armed forces as a whole, and what they stand for. Only once he was a part of it, what did he see? Orcs, Taunka (arguably more aggressive and warlike than the Tauren) and the Forsaken doing shady things, while a faction of Trolls killed their gods to try to avoid the Scourge. (To him, an orc is an orc is an orc. A Taunka is -not- a Tauren and the trolls there were -not- Darkspear, but he had nothing to show him the differences.) The Blood Elves he saw were an occasional magical ambassador speaking to his Orc commanders, and only illusions at that, they instead content to float high above everything in their magical mage city and work alongside the human enemies of the Horde, and Goblins at that point were purely paid mercenaries who would work for the highest bidder and this wasn't an unknown thing....the Horde knew well that Goblins at that time were only working with them because they paid the best and let them blow stuff up.

Its no wonder Garrosh has issues with the Trolls, Forsaken, Blood Elves and Goblins, and that things got so tense with the Tauren when he was shocked at Magtha's treachery. He had -no- social studies regarding the races of the Horde. Thrall and Saurfang imparted upon him the right sense of honor, but he doesn't understand the social intricacies of the different races of the Horde.