Mods: I know I'm not an actual historian, but I felt like I could contribute to this conversation because I am extremely passionate about tanks and own/read quite a few books about them. Tanks are my hobby.

From: TANKS and Armored Fighting Vehicles, Robert Jackson, 2007 Paragon Publishing, Page 189

"The later development of enhanced radiation weapons (commonly known as neutron bombs) would put a different slant on the situation, for armor was no protection against blasts of radiation, and tank crews were at risk of being killed immediately."

Basically, the weapons of the 1950s were the kind that ignored armor. The Leopard 1 was a tank built for a conventional large scale war and the assumption was that a war of that type would involve extremely powerful bombs/NBC threats that would nullify any armor in the first place. The emphasis for main battle tanks of this period tended to favor high firepower and agility.

Edit: Wikipedia says it was also HEAT warheads that made armor seem obsolete at the time but it's Wikipedia so not the greatest source.