X-MEN...why does the phoenix force xtremely powerful?

The Phoenix is not just fire/flame. It's a cosmic entity, one of the powerful beings in the entire Marvel universe. It's pure psionic (i.e. psychic) energy, capable of consuming entire stars to feed itself (essentially absorbing the energy of that star). The Phoenix Force was interpreted differently for the movie... in the comics, the Phoenix Force inadvertently drives Jean insane and she's forced to commit suicide. It's a long, complicated story... If you're interested, I would suggest that you read the Dark Phoenix Saga by Chris Claremont.