Poto
Ancient Mariner
To understand this a bit better, listen to If Eternity Should Fail vs Eternity Has Failed.It's a feel thing. Nicko doesn't play like a metal drummer nor does sound like one. Maybe non-musicians don't understand that, but it's a thing. And is especially important to the rythm section department, in this case, Steve.
A similar situation I can think of is Sting and The Police. Sting performs/has performed Police songs with a shitload of drummers, including Vinnie Colaiuta (who many consider one of the best drummers ever), but they only sound right with Stewart's madness and relentnesness.
Nicko is so essential to Maiden's sound it's hard to describe.