Why do you give Demerol with phenergan?

Demerol is known to cause nausea in most people who take it. Taking Promethazine with it reduces the nausea effect that the drug produces because it is an antimedic. There use to be a pill that had both medications in it called Mepergan Fortis but it is no longer manufactured. No idea why. Even though Mepergan is no longer available, most doctors that prescribe Demerol also prescribe Phenergan with it.