Why did the Framers of the Constitution create the legislative branch with the bicameral Congress?

The main reasons the bicameral Congress of the United States (consisting of two chambers: Senate and House of Representatives) was created are the following:

To prevent tyranny

The separation of powers was aimed to prevent tyranny. A unicameral legislature would mean to put too much power in one institution. However, with these two legislation, each can be checked by the other, and laws must be approved by both before they can be presented for executive approval.

British government model

Though American had gained independence of Britain, they got their inspiration from many of their laws in matters related to their government, including the inclusion of two houses. The British Parliament had two houses: the House of Lords (Composed of representatives of the aristocracy) and the House of Commons (Led with representatives of the people)

To ensure fair representation of every American state

With a bicameral Congress, representation had a balance between statehood and population. Representation in the House was based on a state's population, and representation in the Senate is equal for every state. This accomplishment wouldn't have been possible with a unicameral Congress and would have been unfair to large and small states.