Why do doctors insist on morphine?

I sustained a major injury to my lower back (L5-S1) 15 years ago when someone kicked a stool out from under me.  Since then I have tried everything, including the IDETT procedure in 2000 (just made me worse) and major surgery placing an artificial disc at L5-S1 in 2003 (this completely disabled me).  With improved technology, they have now determined that my pain has never been caused by a herniated disc, but rather by damage to my left pyriformis muscle, which is now totally atrophied and 1/3 the size of my right pyriformis muscle.  I am pretty desperate.  If the pain could be controlled, I could return to work.  I have no symptoms other than pain. My pain specialist has now reduced my muscle relaxant (the only thing that helps me), while insisting that I increase my narcotic (to a massive dose).  The narcotic does very little to help me, while the muscle relaxant enables me to live a near-normal life.  Do all doctors have the same preference for morphine over soma and if so why?  Without the muscle relaxant, I will return to a bedridden, pathetic mess.  While with the muscle relaxant, I am a very functional and happy human being.  It seems so cold and callous to deny access to an approved medication that really helps me, while pushing me to increase a narcotic that does not help at all.