They made a name for themselves and were able to sail on their international popularity based on their placement. It's the whole "right place, right time" kind of thing. I love them, dearly. They just get way too much attention for being a staple amongst so many others that were similar to them. There's not a lot of originality (with the "White Album" being a bit of an exception, I will admit) but that's why their records sold.
There are just too many things that were milked to death in the mainstream; the mainstream is part of the problem as to why nothing particularly interesting ever really stems from it.
It's been said that some of the best art/music/films/books/insert own choice here, is the undiscovered, unexploited thing.
As for the Beatles (I love them dearly as well) individually at various times, were the first to condemn the Beatlemania madness that swirled around them in the early days. George refusing to participate in a ticker tape parade on the second American tour as an example. All four, especially Ringo, stating that they played like crap live by the time they called it quits in 66 from touring even if it was partly the result of all the screaming girls.