If there existed a canon in Apostolic times, why would Marcion publish his canon and argue that *THIS* was the correct list of books?
Because Marcion was a widely acknowledge heretic who said lots of heretical stuff. If you are going to use widely acknowledged heretics as evidence then no Christian theology has ever been established because there have always been heretics challenging orthodoxy as pointed out as early as the book of Jude.
Because he was a very rich Gnostic like so many others...
Marcion's list left out most of Luke and the rest of the Gospels...It left out Hebrews and Revelation...And he left out many verses of what what left...