#4 Mysterio does not have the same kind of star power as Brock Lesnar
Even the biggest names from today like Roman Reigns and Seth Rollins aren't household names like Stone Cold Steve Austin and The Rock were, back during the Attitude Era of WWE. Wrestling is watched by a very select audience in today's day and age and yet one name stands out from the pack, towering over the rest. This man is Brock Lesnar, someone that people know for his accolades across spheres and for his longevity in wrestling.
Brock Lesnar hails from an era when wrestling was a lot more popular, and honestly, so does Rey Mysterio. But Mysterio was a Cruiserweight in WCW, and there was an invisible barrier for these individuals who would be a level below the likes of Sting, Goldberg, Ric Flair, and Hulk Hogan, among others. Even when he came to WWE, even though he's had a career that few can boast, Rey Mysterio never received the kind of push that Brock Lesnar did.
As a result, while he is hailed as a legend in the business, he will never have the box office appeal that Brock Lesnar has, and may not be the right candidate for the Championship.