Double bass or double kick weren't even remotely known in the '70s and the early '80s as they are now. Some drummers used it, but not as extensively as, say, "Animal" Taylor would do on Ace of Spades (or, later, Dave Lombardo on Reign in Blood and Ingo Schwichtemberg on Walls of Jericho); anyways, it was a completely different style. Both of the two most influential hard rock drummers of the '70s, Keith Moon and John Bonham, didn't use double bass.
Also, Nicko started to play in the mid-late '60s in the jazz/blues scene. The drums kicked in (no pun intended) on him when he heard The Dave Brubeck Quartet and by the age of 14 he was already playing in pubs. When he joined Trust and later Iron Maiden, he wasn't a heavy metal / hard rock drummer, rather a blues rock one, and during the sessions for Piece of Mind, he proved with facts that he just didn't need a second kick.
Also I cannot think of many Iron Maiden songs before Nicko that would work with double bass. Phantom of the Opera (between the verses), Transylvania (fast section), Genghis Khan maybe. The opening to Gangland, drawing inspiration from Hot for Teacher as someone mentioned?
Neither after Piece of Mind, to be honest.