Readers often wonder why characters who are nigh invulnerable, like Superman, Zatanna or Spider-Man, would allow themselves to be whomped on rather than unleash their full power.
This question is generally answered with an explanation involving ethics, morality or "because the world/galaxy depends on it", but this kind of explanation only placates readers for a little while.
So why won't these superheroes unleash their full power?
Superman tries to refrain from killing at all costs, and despite all the destruction we've seen in the comics he tries to keep collateral damage to a minimum too, so it makes sense for him to hold back.
Zatanna's powers were passed down from her father, so the only thing holding her back from unleashing the ultimate power within her is her own self-confidence, which can be a hard barrier to break through.
Spidey certainly doesn't lack self-confidence, so what's his excuse for holding back?
He's opposed to killing too, but he also just holds back to give the bad guys a chance in battle, which Doctor Octopus discovered when he swapped bodies with Peter Parker during the "Superior Spider-Man" storyline. Cocky, ain't he?!