"By the time America came about more people were reading and writing and spellings were being laid down in the dictionaries. Perhaps the Americans wanted to standardise (ize) the language to make spelling easier both for native and foreign learners."
But that's true on both sides of the Pond. Johnson's dictionary only came about in the mid-late 18th century and Webster's in the late 18th/early 19th. There were no dictionaries while what was to become the USA was still evolving and growing. So both the English and Americans strove to standardize (-ise) spellings around the same time - they just occasionally chose different options of already accepted spellings.