what is the reason behind that phellem is made up of dead cells

Dear student,
Phellem or cork is made up of dead cells due to the activity of cork cambium which does periclinal divisions and cut the suberised dead cork cells towards outside.Thus,this suberised cell wall which is impermeable to the gases and other substances act as an insulator to the inner cells of the plants and also provide extra protection to them.
Regards