Why was Sandman picked first by man in moon as a guardian? why not the others?

Sandy is very powerful. He controls sleep and dreams. Before the guardians life was obviously hard. Dark Ages, The Plague, War, Salem Witch Trials. And really there isn't very positive or hope in those eras other than towards the Gods and Goddesses or God himself. Almost as if there was nothing for anyone to look forward too or feel safe from. Sandy would be the first because he sparks that hope that wonder that change that led to many kids not being so frightened of the dark. Towards the end of the movie you do see how his power truly binds the belief in the remaining guardians. Otherwise we'd all be living in nightmares, and worry, and fear.

I don't know just seems logical to me that Sandman was first xp.

MiM (Man in Moon) need some help watching over the children of the world. So, he chose the first guardian, Sandy (Sandman), to help him out