Here's why:

* Isomorphism means two substances have the same crystal structure. While both NaCl and KCl have a face-centered cubic (FCC) structure, their ionic radii are different.

* Sodium has a smaller ionic radius than potassium. This difference in ionic radius leads to a difference in the lattice parameters (the dimensions of the unit cell) of their crystal structures.

* Although both structures are FCC, the distances between ions in NaCl and KCl are different due to the different ionic radii. This difference in distance affects the overall structure and makes them not truly isomorphic.

In summary:

While both NaCl and KCl have the same crystal system (cubic) and similar structures, they are not isomorphic due to the difference in ionic radii and lattice parameters.