Conductometric titration of Strong Acid And Strong base
You correctly identify that the number of cations do not change in solution, so from a charge perspective it appears that there should be no significant change in the conductivity.

However, consider two things:


Sodium ions are much heavier and larger than free protons and so should travel through water more slowly. Charge moving more slowly
means lower conductivity.
In addition to their smaller size and lightness, Protons can also "hop" from water molecule to water molecule and so can move through the solution much more quickly and easily. Other water molecules do not act as obstacles


Point 2 is called the Grotthuss Mechanism.