Supposedly he had been hurt very badly and needed the life support
the mask supplied. (Remember the back of his head in "Empire?"
Some friends and I decided he looked like an ice cream cone.)
My memory fades... but I seem to recall he was hurt during the so-called
Clone Wars that came before the time of the Star Wars Trilogy.
P.S. Doc, Grumpy, Sleepy, Happy, Dopey, Bashful, Sneezy