Fallout 3 is popular because it had a massive advertising campaign. It's popular at the moment, but it's ultimately an average game that doesn't bring anything new to the franchise. It's not pushing any gameplay boundaries or touting any original features. It doesn't do anything that hasn't been done before, or done better.
Fallout 3 will be forgotten once the next shiny new thing comes out, while Fallout will still be remembered as the classic that it is.
Oddly enough, the harshest critics of World of Warcraft say the exact same thing (In fact, I saw some of them over in the Fallout MMO thread yesterday). That it didn't add anything new to gameplay, or that it was average. It doesn't really matter though. Wow has sold 10 million + subscriptions and if you ask someone on the street if they've ever heard of an MMO I'd be willing to bet that the most common response, if any is WoW.
Furthermore, I think that Fallout 3 is much more than an average game (I also thought Wow was above average, but people either love WoW or hate it). It isn't perfect, but then again neither were Fallout 1 and 2. Games don't have to be perfect for them to be remembered fondly by the people who played them they have to be fun and Fallout 3 has that in spades.
Finally, yes people that play consoles move on to new and exciting games when they are released, but believe it or not, people that own consoles are not all 5 year olds with ADD that forget everything when they see the next shiny. Heck even people with ADD carry their previous experience forward to judge their new experience. And when Fallout 4 is inevitably released the vast majority of people that have previous Fallout experiences will have that experience with Fallout 3 exclusively. That vast majority of people who have played any Fallout game will use their common experience of Fallout 3 to set their expectations for the future.
It doesn't mean that Fallout 3 is the best of the series (it could be the worst), it doesn't mean that Fallout 3 defined the lore, but Fallout 3 will be the definitive game because it is the game that most people who know anything about fallout know. It isn't a matter of fairness or correctness, it is a matter of convenience.