And then we throw in your spellchecker 
Aaugh! Those aren’t even homonyms!
(At first I didn’t even spot the error, and thought you meant that the default spellcheckers in MS Word, etc., are set to US English.)
Not sure we can say for sure. But note that his spelling books had much more influence than his dictionaries. That may be the main reason right there. Also note that the spelling changes he wanted changed over time.
Here is the NGram Viewer chart for /color, colour/ in American usage. Americans preferred “colour”, until about 1830, when “color” began to appear, and within about 30 years, the changeover was complete.
Hmm… actually we call it “the Queen’s English”, or “King’s English”.
Its not a strict rule, no need to cite counter examples.
As to weather Webster was the responsible person ,
I’d say so, because when Carergie’s mob tried to simplify spelling, they did check through the three most popular dictionaries for the USA, for the current popularity of their first 300 words to official declare (if it succeeded , it might have then become known as Carnegie’s English, Roosevelt’s English or The Presidents English… )
They found that the later dictionaries had more of the Webster reforms than the older Webster dictionary, see the following URL, nytimes 1906.
I suppose they counted the change of the suffix “ed” to “t” as one word, so as to keep it from making the statistics look bad. Thats one reform that won’t be mist.
http://query.nytimes.com/gst/abstract.html?res=9E03EEDC1E3EE733A2575BC1A9609C946797D6CF