Maiden has never used sample replacement live. In the studio, yes- but never live. As others have said, the triggers are to automatically open and close noise gates.
This is true, but another important factor to consider is
how the drums are headed and tuned. If you listen to Nicko playing Maiden songs on a smaller kit, it still sounds notably more "lively" and diverse in pitch/tone than Simon's kit. Here is Nicko playing Trooper on a kit very similar in size to Simon's (his kit here has 10", 12", 13", 14" toms, 18" floor, 24" kick. Simon's kit has 10", 12", 13" toms, 14" and 16" floors, 22" kick):
Nicko traditionally uses single-ply heads (either coated or clear with a center dot) and tunes his drums medium-high and wide open with no muffling (save for a small pillow in the bass drum). In a live setting, this can create issues with overtones and extra resonance, but it makes it
much easier for the audience to differentiate each drum because the sustain of each drum is more pronounced.
Simon is using double-ply coated heads, and is tuning his drums significantly lower. Not only does this contribute to the tone of each tom sounding more similar to its neighbors, but it makes it harder to discern the difference in pitch because there is very little sustain, and what is there is lower in pitch and more difficult to discern over the attack of each hit.