Why is Iron Man/Tony Stark naming his suits as "Mark 1, Mark 2, Mark 3"?

"Mark" used in this way isn't a name; it's a synonym for "model" or "version". According to Wiktionary, the relevant definition is

A particular design or make of an item (now usually with following numeral). [from 15th c.]

A famous usage of this nomenclature was in the naming of the Colossus Mark I and Mark II computers, used in code-breaking during World War II.