Why Did Ferrari Wait So Long To Race Phil Hill in Grand Prix?

Wolfgang von Trips raced in the Argentina Grand Prix immediately after joining Ferrari for the 1957 season. So why was was his teammate Phil Hill relegated to sports cars for so long? It wasn't until after Hill borrowed a Maserati for the French Grand Prix that Ferrari relented. (Hill is shown above testing his seat for that race.) Even then he may have promoted Hill only because Luigi Musso died in that race, leaving a slot open.