When RPM comes down, so does the airflow through the carburetor.
With less airflow, there is less pressure drop across the venturi, assuming fixed throttle.
Manifold pressure (on an unsupercharged engine) is atmospheric (or ram) pressure minus the venturi pressure drop.
Yes, there are additional factors like the supercharger, air filter, intercooler etc. but the essentials of your answer are per the above.