Why not place the combined fleet under a Spanish admiral?

Did Napoleon's need for personal prestige prevent him from putting a Spanish admiral in charge?

That was a real constraint, but his appointment of a Dutch admiral to command the barges complicates the explanation.

A viewer argues that Napoleon's recent accession as emperor made personal credit for victory politically important, making Spanish command of French ships difficult to accept. The reply acknowledges this constraint but points to his use of a Dutch admiral to command the invasion barges. That appointment suggests that foreign command was possible in some circumstances. Napoleon would have been present to claim credit for the crossing himself, so the two cases were not identical.