Why, then, did the Constitution create a bicameral legislature? Why not merge the House and Senate into a single unicameral legislative body?

There are three main reasons...
The second was more theoretical. The framers' emphasis on the idea of checks and balances led them to be suspicious that a unicameral legislature might consolidate too much power in one institution. By dividing legislative power between the House and the Senate, the two chambers would serve as checks against each other's authority, theoretically preventing either from ever gaining tyrannical power.

3. tyrannical power

In this spirit it may be remarked, that the equal vote allowed to each state, is at once a constitutional recognition of the portion of sovereignty remaining in the individual states, and an instrument for preserving that residuary sovereignty. So far the quality ought to be no less acceptable to the large than to the small states; since they are not less solicitous to guard by every possible expedient against an improper consolidation of the states into one simple republic.