Dionysus was a mortal that was raised to divinity. While he is most well known for wine and revelry, his portfolio also grew to include:
* festivals
* fertility
* the wilds
* crops
* sanity
These are centrally important to the daily lives of average citizens for the time. Sure Zeus is the king, and sure Poseidon is incredibly powerful, but I need my crops to grow, sons to help me run the farm and carry on my name and lets face it an opportunity to drown my sorrows once in a while.
Orphism espouses the idea of an immortal soul and in a similar fashion to Hindu reincarnation the goal of the follower is to exit a cyclical pattern of death and rebirth in human bodies to become one with the gods.
Who better to worship and lead such a 'religion' than a man who became a god himself.