I know this post is a couple of years old, but I just read it and wanted to continue this topic of such a fascinating vehicle. I know these vehicles were unique to the Marine Corps, and according to Military Wiki, The Marines wanted a small, light weight vehicle that could be carried by helicopter of the era and the Mighty Mite fit that bill. The Mighty Mite was never in direct competition with the M151, rather the M442 was designed for a role that the M151 was not originally intended for. However, by the Vietnam war era, helicopters had become much more powerful, and were capable of carrying heavier payloads, such as the M151, rendering the need for the M442 obsolete in that role.
One other factor may have been in play to the demise of the M442. During the 1960s Former Ford Executive Robert McNamara became U.S Secretary of Defense under John F Kennedy and Lyndon B Johnson, and played a major role in the escalation of U.S involvement in the Vietnam war. As Secretary of Defense, McNamara took a page from the Ford standardized parts supply playbook by standardizing equipment within the branches of the military. For example, weapons unique to a military branch were either phased out or standardized across the board. This probably applied to the M442. Only the Marines operated these little vehicles. The Army had no such requirement for a small, air deployable vehicle at the time. So it's possible that the Mighty Mites were cut and replaced by the M151, which of course were cheaper, used by all other branches of service and could be air deployable by the availability of more powerful helicopters.
Ed
