Chuck Mangione was also on during the "Feels So Good" days.  I have to imagine Herb Alpert was on during his resurgence with "Rise." Add that to Al Hirt, Clark Terry, etc. and it clearly wasn't anything to do with a moratorium on trumpet players.  I imagine if Doc had suggested Maynard he would have been on. Maybe he was the one trumpeter that Doc felt could have "shown him up."  We all know in reality that Doc was a far superior player in terms of all around trumpet playing, but Maynard created a level of excitement that certainly would have overshadowed anything else going on the bandstand that night.