The explanation given is that director Marc Webb had been stuck in a contractual tug-of-war between Sony and Fox over work on this film and a sequel to.
In-universe, Peter kept his refurbished original suit on hand but made his costume from The Amazing Spider-Man 2 his primary outfit.
The Amazing Spider-Man 2 is a big let-down over the first film of this abruptly ended series.
Peter and Gwen's entire romance becomes this when she gets killed by the Green Goblin.
Their venom is dangerous when used by anyone else, and when Harry takes it to cure himself it ends up turning him into a giggling, violent psychopath.
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