I know it's anecdotal evidence, having seen Iron Maiden at least 15+ times live since the late 80s, one thing you notice today that on their live shows there are a huge number of young teenagers. Their shows are not filled with old fans like myself. So there is this new wave of Maiden fans that probably don't even know what it is like to buy a CD. They did establish a strong reputation, sure, but their fans are not only the old ones form the 80s.
Also, unlike other big 80s bands, they don't sue their fans for downloading. They also didn't hop on every possible trend in music, they kept true to their origins. They also didn't became born again christians who refuse to play at festivals because some 'satanic' band plays. They also didn't create drama. When members left or where fired, it wasn't publicized and criticized by other members.
Despite making some of the most memorable heavy metal music, Iron Maiden was always ran as a business, since their early days. And this focus allowed them to go through the download era without issues. They never had major video exposure on MTV (yes, back when there was music on that channel) so music downloading didn't really impact them.
Up the Irons!