The most basic site in the reactants is the pyrrolidine nitrogen atom. Its conjugate acid is shown on the left above. This "onium" ion will be in equilibrium with many other conjugate acid species, one of which is drawn on the right. Equilibria of this kind always favor the weaker acid and base.
Since the tetravalent nitrogen in the "onium" ion cannot form additional bonds, this species is unlikely to play a role in the reaction. Although the conjugate acid of the ketone is present in much lower concentration, it is able to bond to nucleophiles, and therefore is a better intermediate for subsequent reaction.