Sometimes it's a matter of atmosphere as well. A nude massage is considered to be more restrictive as well. No touching, just lie there and let her do the massage. While B/S's and reverse massages usually allow you to caress the ladies as well (restriced on certain spots of the body, obviously).
One of the ladies I was with mentioned that when she is asked to do just a nude massage, or even just a topless massage, she feels a little uncomfortable because she feels more restrained in what is allowed to take place. So she does not feel as relaxed as she normally would.