9-ball and 10-ball are better for spectators because the next object ball is dictated by the rules. Spectators don't need to know much about playing to follow the game and see the next shot before it's made.
8-ball adds variables for additional strategy. It's often more challenging to play safe, since the opponent has up to seven different object balls to shoot at. Using all fifteen balls means the table is more crowded. Consequently, the player with the fewest balls on the table is basically losing until they have a legal, makable shot on the 8. Dropping six balls and missing the last remaining ball of the set (i.e., solids or stripes) is a great way to lose the rack. Not every player has the discipline to refrain from dropping any balls until they see the potential to run out in a single inning.
The very aspects of 8-ball that make it more challenging to play make it less fun to watch. Only good players can see the potential runout sequence, and even they often see a different sequence than the pro at the table attempts.
