You ask why more sites use American spelling than British spelling. That's probably because the sites you are looking at are either written by Americans, or for Americans, or both. But that doesn't have anything to do with what's right for your site.
For your site, you've got to use whichever is most natural for your audience. If the audience is truly international, then use whichever is most appropriate for the nature of the site. For example, as a journalist, I write articles for American magazine publishers. I always use American spelling, and I try to write in an American "voice". But in my own blog, which tells of my experiences as a resident of Edinburgh, I naturally use British. In this forum, I use British spelling because the audience is completely international, and I don't see any reason to favour one over the other.
Also, please stop using the word "English" as meaning something other than American. As I tried to explain earlier, American and British are both forms of English - as are many others. American and English are not mutually exclusive.