For your information, the Americans did not change it. Neither did they change "colour" to "color". In Spanish, to realize is "realizar", as is virtually any other word ending in "ize" in American English. I believe the British lean more toward French etymology where the Americans lean more toward bypassing the French and going straight for Latin. Of course, if you're still looking for an answer as to why that is, I guess I could finger Noah Webster, who put it into print big-time back in the early days of an America finding its own way in the world.
And by the way, "marry", "merry", and "Mary" sound identical to me. So do "wail" and "whale", but not "well". But "chock" and "chalk" are very noticibly different, as are "dawn" and "don". It actually bothers me when someone pronounces them to where I can't tell the difference.