A fairly simple answer I'm afraid, but, essentially, the horses weren't big enough to make effective cavalry mounts, but the could pull a wheeled vehicle reasonably quickly and well. Once horses had, through selective breeding, gained size and strength, they became suitable as mounts. There are interesting Assyrian reliefs that shows two riders operating together, one using his bow, the other controlling both horses, rather like a two-man, two-horse chariot, but without the expense of the vehicle. http://en.wikipedia.org/wiki/Cavalry#mediaviewer/File:Assyriancavalry.JPG Once riders could use weapons and control horses, true cavalry was born. with the appearance of true cavalry, chariots as war weapons become obsolete. Britain continued to use them until Roman times because of the small size of British horses.