Why does the US have their own variation of English (differences including ‘color’ instead of ‘colour’ and ‘urbanization’ instead of ‘urbanisation’)?

See

• Why do Americans use the spelling color and British use the spelling colour for the same word?
• Why do some people believe that the ‘ISE’ suffix is English and the IZE suffix is American?

There have been many proposals for spelling reform in English over the years, and in fact the variant spellings color and –ize have been around for a long time. (They reference Latin and Greek respectively, whereas colour and –ise are taken from French.)

As discussed in English-language spelling reform – Wikipedia, the spelling reforms proposed by Noah Webster had the most success of any, but they only succeeded in the US. Not all of his proposals were accepted: see 26 of Noah Webster’s Spelling Changes That Didn’t Catch On