The JR Rail Pass has never been allowed on the Nozomi. There are two likely explanations for this.

1. Back when the pass was first being sold, there were many fewer Nozomi trains per hour than there are now, and the Nozomi was marketed as a special train. As the number of Nozomi trains per hour increased, there was no attempt to change the rules of the pass.
2. JR Central vehemently protects against any attempt to cut into their revenue from the Tokaido Shinkansen, of which the Nozomi makes up the vast majority. (For example, Nozomi tickets cost more than those for Hikari or Kodama trains for the same distance.) They see no reason to make travel on the Tokaido Shinkansen any cheaper than it is now given that Nozomi trains are generally full most of the time.

I personally think #2 is the most likely. There are plenty of other passes that cannot be used on the Nozomi. On the other hand, passes like the JR West Rail Pass can be used on Nozomi (but not on the Tokaido Shinkansen portion) and Mizuho trains, and there is no restriction for the fastest Hayabusa trains on the Tohoku Shinkansen, so the policy is not consistent.