What reason would have prompted the designers of the articles of confederation to exclude the role of a president for the nation?

The designers of the Articles of Confederation excluded a president because they worried that a president would be too similar to a king.

Americans feared a King because the last time they were controlled by the King, it didn't work out. One of the most significant reasons behind the United States decision to declare independence from Great Britain was the tyrannical actions taken by King George III. This includes punishing the colonists, taxing citizens without their consent, etc. This is why America's first constitution did not have an executive branch (aka a president).