I think it had a lot of momentum going from the name of the series itself. Just look at how well Resident Evil 4 sold (especially on the PS2). When people heard of Resident Evil 5, a lot of people went crazy for a sequel to such a big franchise. This sort of craze was also evident in games like Final Fantasy XIII and Devil May Cry 4 (along with a variety of other franchises). Many people wanted to see old franchises upgraded with better graphics and the features possible with HD consoles. And since Resident Evil 5 was part of such a classic franchise, word of mouth likely spread over to a lot of people who had never picked up the franchise, but wanted to know what all the fuss was about.
But mainly, it hit the right demographic and it had a lot of hype.
