I would imagine that your teacher and or text has a specific answer that you are expected to come up with. It may not match mine. Please be careful.
In my opinion, there are two major reasons why this was.
First is the fact that there really had been no modern example of a democracy before the United States was founded. Because of this, most people naturally looked to the elites of society for leadership. This was how things had always been.
A second factor is the fact that there was less education and less leisure time for most people in those early days. Because of this, only the elites had the time and the knowledge needed to become political figures.
Some would argue that our countries are still oligarchies. If you did an analysis of the wealth of all the people in the senate, I am pretty sure that it would be way above average. Does this suggest something? I'll let people judge for themselves. There is a sociological answer as well. With the lack of technology and communication, you really cannot have a democracy. There are certain limitations due to the lack of infrastructure. So, even if things were oligarchic, this was a huge accomplishment in view of the many challenges. Think of how impractical, actually impossible it would be to have a full democracy without infrastructure.
Here's an addition to the first answer; the elite of the New World (Canada and the U.S.) had the means to travel to various cities for political meetings and lawmaking. The "commoners" of the colonies and settlements did not. They were extremely busy with the hard, everyday life of farming, trading, and building and could not have afforded the time or had the financial resources to leave home and spend time governing. Additionally, some of the colonies were originally settled as religious communities which were governed by religious leaders, who in those days were better educated than most colonists and often wealthier.
One last note--most of the settlers and colonists were used to the elite, educated members of society being in control. Many of them emigrated from nations that were monarchies or oligarchies and had no idea how to participate in government themselves.