NaCl structure is more stable than the CsCl structure because of the following reasons:
1)The electronegativity increases from left to right in the periodic table and decreases from top to bottom.
2) If the difference between the electronegativities of the elements are more, then they are said to be more unstable. If the difference between electronegativities between the elements are less, then they are considered to be more stable. The difference between the Cs and Cl is more, hence they are unstable. The difference between Na and Cl is comparatively less, as a result they are stable.