#3 Not enough depth on the roster
SmackDown has some top talent on its roster, especially after the superstar shake-up in April. With the likes of AJ Styles, Samoa Joe, Shinsuke Nakamura, Daniel Bryan, Randy Orton, Charlotte Flair, and Asuka on their brand, SmackDown has the potential to be the best wrestling show on TV.
However outside of the top tier of talent just mentioned, SmackDown lacks the depth to justify going to 3 hours. There would have been a good argument for going to 3 hours if there was no brand split and Superstars could move freely between both brands. But shorter rosters mean that it is more likely shows will be filled with matches the audience has already seen and therefore dilutes the product.
As is the case with Brock Lesnar, the fact that he rarely appears gives his matches a more meaningful feel. With 3 hours to kill every week on SmackDown, it is less likely that we get that meaningful big match feel on WWE's events or at WrestleMania due to Superstars facing each other more regularly in order to fill time on each show.