Why sodium has strongest tendency to form electrovalent bond? Why not Cesium?
Well, they are both alkali metals....
...and therefore they have a marked tendency to be oxidized to their cations...and if we looked at the data, we might find that lithium metal, another alkali, is more prone to oxidation than both of them.
And of course there is the issue of cost. Sodium and lithium metals are dirt cheap...and they have relatively low molar masses...on the other hand, caesium is more of a specialty chemical, that is used (carefully) for special experiments.