Where his peers (Justin, Beyonce,) and in the 90's (Brandy, Aaliyah) switched it up with every album, they had a bigger new sound, new image, new choreography, new visuals,...Usher stayed stagnant and basic.
Usher hasn't done ANYTHING groundbreaking musically or visually in his career, his biggest sound was the crunk movement and that was out a whole 2 years before "Yeah" came out and died a year after, I mean if you call singing over a crunk beat instead of rapping over it innovative and the A Town stomp iconic.... I dunno what to tell ya

In 5 years from now we'll be talking about how artists contributed to the game, what risks they took, which albums were ahead of the curve, Usher wont be in that conversation