It's not too hard to see why the game sold so well. It's actually getting pretty decent reviews, unlike:
RE Survivor: 42.1% on Gamerankings
RE Survivor 2: 52.5% on Gamerankings
RE Dead Aim: 66.3% on Gamerankings
In other words, it's basically the first GOOD RE rail shooter - something fans of the series have been waiting over a decade for.
"'Casual games' are something the 'Game Industry' invented to explain away the Wii success instead of actually listening or looking at what Nintendo did. There is no 'casual strategy' from Nintendo. 'Accessible strategy', yes, but ‘casual gamers’ is just the 'Game Industry''s polite way of saying what they feel: 'retarded gamers'."
-Sean Malstrom