Bruh, most of Robert's Rebellion is better than anything that happened in GOT the series. A series on that would have done numbers regardless.
It has no source material. HBO would have been crazy to start there instead of with the actual book that GRRM wrote. Remember the original GoT pilot was so bad, half of it had to be reshot. And that was WITH a book they could adapt scene for scene. Show would have been straight up canceled if D&D would have started with the Rebellion, making up all of the scenes, dialogue, and characterization on their own.
This isn't even getting into the fact that GoT was seen as a risk in 2011, which is why the budget was modest and didn't allow for any battles. How you gonna do Roberts Rebellion with no battle budget?
Starting with Robert's Rebellion also would remove the mystery from R+L=J.
No matter how you cut it, HBO would have been crazy to start the show there.