what would have been the effect of having so many landowners and college graduates get together to create the constitution

The Constitution of a country determines the laws and statues by which the citizens of said country will be under, and obligated to follow.

The process of making a constitution has varied depending on the country, however, in general terms, the persons that are responsible for making a constitution will contribute directly to the laws and statues.

If only a specific group of persons (ex. landowners) got together to make the Constitution, much of the laws and statues would be in benefit for this particular group only.