It seemed reasonable, I suppose. It probably seemed intuitive that heavier objects fall faster than lighter objects. Of course, science has demonstrated our intuition as being wrong on several occasions.
It was more than just intuitive. It was supported by observation.
Precise timing was difficult in those days, so Aristotle tested the idea by dropping objects through water. The water slowed down the fall enough for him to observe carefully. And sure enough, heavy rocks would sink at about twice the speed of rocks half the size.
If you're focusing on the "why", it's probably because of the Dark Ages in which no new scientific progress was made and the Church taught Aristotle's natural philosophy (what would later be known as science) as if it were fact. Due to religious influence and Aristotle's role in early physics, no one really questioned his work (especially not gravity) until Galileo Galilei.