The United States under the Articles of Confederation did not have a central executive officer, instead having a administrator of the Confederation Congress with little power over the body. You have to understand, the United States as we know it, a group of connected states under a single national government simply did not exist under the Articles. There was no "national" government. The United States in Congress (the other name for the body) barely existed with no one questioning that the body was impotent against any single state. When the Constitution was ratified, it created the government we know, and it also vested in the hands of the executive branch the position of President as the head of state and chief executive of the branch. This is why George Washington is the first president and not John Hanson: Hanson was simply the guy who got to babysit the (up to) 13 delegates while they attempted to conduct Congressional business. Washington had a country to run as the chief executive.

I think the general confusion comes in that both positions were titled "president" despite having such drastically different levels of authority.

edited: I originally put "pretended" to conduct business, but felt that since they did accomplish some things, that attempted would be more accurate and less opinionated, as they weren't always effective in their business (which is what led to the Constitution being created, an illegal act under the Articles)

A similar question was recently asked in this thread. Not to discourage further discussion, but this may point you to what you want to know.