Because during Dickinson's first stint with Maiden they failed to make any kind of attempt at truly evolving and changing their sound.
And GreatOne, don't all these song have a guitar intro of relatively similar length, a solo that doesn't change all that much form song to song, and Bruce screaming at the end?
As for the difference between Piece of Mind and FOTD, yes they sound different because Janick's guitar playing and Bruce's voice changing a bit but the song formula is still pretty much the same.