Why didn't Lucifer turn Sam into a demon?

So Sam jumps into Lucifer's cage in order to prevent the apocalypse and seal Lucifer and Michael forever. Cas gets his body out somehow but his soul stays behind. We know time is much slower in hell, with Dean having spent around 40 years in there despite the short time on earth. It's possible time is even slower still in the cage but I don't think that's confirmed. We know that Lucifer created the first demons, that demons are made by torturing souls in hell and that Lucifer tortured Sam in the cage the equivalent of 120 hell years if not more. Sam, at this point, already had many demon attributes and had been conditioned with demon blood. So why didn't Sam become a demon? It seems like all the pieces were there. Is there an actual reason given or is this just a plot hole? It's implied that Dean was already beginning the process, with him taking up the role of torturer as the first big step, and he was in hell for a comsiderably shorter period. He was also in normal hell and not the cage. Plus I imagine Lucofer is a much better torturer than any demon.