Why only ketone forms enolate in Claisen condensation between ester and ketone?
The pKa of a ketone alpha hydrogen is of the order of 20, the pKa of an ester alpha hydrogen is of the order of 25 source. Bear in mind that this is a log scale, meaning the proportion of ketone enolate to ester enolate is 100000:1.

In answer to the comment by @orthocresol: the ketone does self-condense, but this is reversible. The condensation with the ester is irreversible due to formation of the anion of the product B-diketone.