We all have Webster to thank for this difference in the two types of English. Shortly after the Revolutionary War (or the War for American Independence, as you Brits call it) Daniel Webster felt for us to be truly free from England's rule, we needed to create our own language. Knowing that it would be impossible for us to completely change languages, he removed a lot of the "useless" letters in the spelling of words, for instance the "u" in colour to make into color. He also reversed the french influence of "re" to "er," like theatre to theater. There are also a bunch of little things too, like replacing "i" sounding Ys to Is (tyre becomes tire).
Most people of the time thought Webster as being nuts, but he was very good at selling his idea to schools and libraries. So a generation later American English took hold.