As Mark being the earliest one of all the gospels, why doesn't it mention the virgin conception?

This haunting question came to me after seeing a documentary regarding the gospels and how John is not considered synoptic. It seems that scholars hold the belief that Mark was the first gospel ever written making it not so far from the time of the resurrection and so I personally think that Mark must be the most reliable. Looking into why Mark doesn't contain the virgin conception, I stumbled across scholar findings regarding other sources other than Mark that were responsible for both Matthew and Luke. For example, the Q-source and the M and L source. I don't know if I believe these theories but since Mark is the earliest, it sure does make sense. But my question still stands why didn't Mark contain the virgin birth given that it was the very first gospel? Why didn't any of the disciples have a more detailed account of the life of Jesus for that matter, I mean I think I would have given up everything to have wrote everything with every detail as possible if I was one of the disciples, wouldn't you? One part that makes me think about this, is when Jesus told his disciples who He was but didn't want them to let any one know until it was all done, if this was the case why didn't they mention each and everything once everything was done.