This is a sensitive subject. But probably, in forced oral sex, the victim is afraid of being strangled if he/she uses the teeth. Just saying, that fear could prevent them from thinking of something like that as a means of fighting back.
If someone has their hands around you neck or is beating you in the face if you would bite it would only get worse.Also it isn’t mentioned in this case but in many there are weapons involved. I don’t know about you but if someone has a gun to my head I’m not sure biting would be the best move?