Garrosh wanted to build a wall and make the gnomes pay for it!Garrosh's arc was meant to demonstrate that event the most docile creature can become a genocidal tyrant given enough motivation.
Sylvanas' arc can be interpreted as "Even the most genocidal tyrant can know restraint."
Garrosh came from literally no where lore wise. Sylvanas was, even from warcraft 3, well loved and well characterized.
Garrosh's motivation is "Make Orcs Great Again" (I am not even kidding even if Trump was not a thing i would of typed that exact phrasing)
Sylvanas' motivation is Revenge.
But the real reason why: Everybody hates the new kid on the block.
Everybody hates people promoted via blatant nepotism
Everybody hates people promoted via blatant nepotism who are not good at their jobs.
... ... ... ... ... ... ... <insert trait of garrosh>