why did the framers of the constitution create a presidential rather than a parliamentary system of government, and a bicameral rather than a unicameral legislature?

1st Question: 

The Framers wanted a division among the branches of government, in which some power was given to the judicial branch, some to the legislative branch, and some to the courts. Compared to a presidential system, the legislative branch in a parliamentary system has much more power.

2nd Question
The framers needed to find a way for representatives to fairly represent both population and states, meaning that although state population affected representation, it wouldn't be so much so that small states like Rhode Island had almost no voice. To do this, the created a bicameral legislature, with each state having equal representation in the Senate and the House having numbers of reps based on population.