That’s because you are farming with a raid group I suppose (since Aggramar is nearly impossible to solo).
The mount has a roughly 0.8 to 1% drop chance (as all do), but the thing is, that you have to divide this drop chance by the amount of players in the raid group since only 1 can drop at a time. I was personally farming it with full raid groups in BFA (around 190 attempts), but due to the abysmal drop rate, I just abandoned the farming and will wait until I can solo it. Unfortunately legion content is screwed from the SL scaling, and it looks like it won’t be fixed any time soon, so meanwhile I will just wait for some time to pass by, and give it a go in 9.3 
I suggest you do the same if you don’t want to spend extra time. At the current time, it’s just worthless to gamble for such a low drop chance.
