It's because he lacks the defense that most other high kage level shinobi possess, He has great offense but should an attack slip thru his guard he can be quite squishy.I fail to understand how Deidara isn't high kage, when ranked.
ppl such as the uchiha have things such as kamui, or susanoo, and Hashi basically recovers from most physical damage, and Naruto and Bee have their bijuu cloaks which can tank all but the strongest attacks. This is basically the only thing holding Deidara back from being high kage level imho.