At the risk of going over old material, and as Retro has pointed out, the trend was started in 1957 with the incredibly low (for the time) Mopars, such as the Plymouth Fury Hardtop. These images are taken from the January 1959 issue of Popular Mechanics and I think explains the reasons why they started to get so low. Some of the problems associated with it included the chassis used by most US manufacturers with exception of Rambler - they didn't have a chassis to get in the way of foot wells. Europeans had already gone to monocoque bodies, so could continue with cars getting below 54" which at the time seemed as low as you could go. This issue was also their Annual Auto Show edition, and there is a great article "Why do our cars look the way they do?", with questions asked of Ford's stylists George Walker, Elwood P Engel and Robert H Maguire along with Chrysler's Virgil Exner and Bill Schmidt. George Walker said "How many times a day does a person get in and out of a car? We say about four times. Why would he forsake good silhouette just because he knocks his hat off a few times a day."