I was under the impression it was partially due to him feeling the ‘U’ made the words feel “too French”, and he supposedly really hated the French (apparently he was a bit of a fan of the ol’ racism), but mainly he wanted something new and uniquely American - and English spelling was a even more regional, wild and inconsistent back then, so he hoped to simplify it, bring it some consistency, ideally whilst also telling the French to piss off.