I'm strictly speaking from a European/UK point of view.
It was #1 in a number of European countries as well, including the UK
http://en.wikipedia.org/wiki/Iron_Maiden_discography
http://en.wikipedia.org/wiki/The_Final_Frontier#Chart_performance
Mainstream popular music has always (and I do mean always) a mass of quickly produced titles designed to appeal to the single buying demographic (which is still to this day pretty much 13 - 14 year old girls and has been so since the '50s).
Go here....
http://www.everyhit.com/retros/index.php
...pick a year and a week and see how many of the singles you recognise. The bulk is throwaway stuff that is just forgotten about now.
That's why we think music from (insert chosen year/decade) is better is because the 'good' stuff gets remembered and the dross forgotten. The same with hold true of this decade when its looked back on in 10/20/30 years time. The funny thing is that a lot of what gets remembered was either never released as a single or never charted.
Take Bob Dylan as an example, 'Blowin' in the wind' is seen as a classic of its day and most people imagine it must have gotten huge amounts of airplay. However the truth is that it never even charted.
http://en.wikipedia.org/wiki/Bob_Dylan_discography#Singles
Oh, here's what was in the UK charts the same month as it was released...
http://www.everyhit.com/retros/inde...1962&m1=08&y2=1962&m2=08&sent=1&day1=1&day2=3
Most of Dylan's stuff did poorly in the charts, and serves as a good example of this effect.
You can see the same with 'The Stone Roses', a great band that is synonymous with the late '80s and early '90s indie scene in the UK, however once again singles chart success did not occur, while album sales were better.
http://en.wikipedia.org/wiki/The_Stone_Roses_discography
How about Black Sabbath, they pretty much invented Metal and everyone knows a Sabbath tune or two (play Iron Man or Paranoid to just about anyone and they will recognise it after all), they must have done OK....
http://en.wikipedia.org/wiki/Black_Sabbath_discography
...however with 18 studio albums, 5 live albums and 44 singles released they managed only one #1 album and the highest placed single (Paranoid) peaked at #4 in the UK and #61 in the US (it did better in Sweden), and of the 44 singles it was the only one from the first five albums that charted in the UK!
Ignore what's in the singles charts today, on the whole its as poor as what was in the charts in any year/decade. Good stuff is around, a hell of a lot of it as well, just don't expect to find much of it in the charts (as you never will and that hold true for the past as it does today).