What is the possible reasons people join cultic groups?

Greed and lust for money is the root cause of all evil. Also, because they are mad at GOD, because of unanswered prayers.

Answer

Depends on the type of Satanist.

"Traditonal" Satanists become satanists because they reject God and hope that rooting for the other guy might help them out.. could be greed, could be fear, could be boredom...

LaVeyan Satanism is more of a "mystical philosophy", in that there are magical rituals, but they do not believe in (or worship) a literal devil, using a Miltonian style Satan as a sort of mascot.