I think that LoS sold better because it was riding on a serious hype train. I'm pretty sure MANY CV fans bought it. Not every one of them liked it though. there's a reason it was divisive.
But it was covered by more mainstream gaming websites and the like, hyped up to be the big blockbuster return to Home Consoles for Castlevania.
Once that was all said and done, MoF therefore had a much tougher time.
First of all, it is for a relatively new console. LoS came out pretty well into the run of the Xbox 360/PS3 era. both were pretty well established by then, having been around for 5/4 years, respectively. The 3DS has been out for 2, so far, and had a pretty weak launch. the 360, along with the PS3, are generally considered to be the "hardcore" consoles among mainstream gaming culture, while the Wii is the "family" console, and the handheld Nintendo consoles are considered the "kid" consoles. A NEW kid, console, no less. Then the fact that people who felt burned by the first LoS were obviously going to pass on this. Some people who maybe did not like it going 2D, didn't think it was worth their time. then the fact that once it came out, it was far below LoS' quality in terms of pretty much everything short of atmosphere and story.
LoS2, however, is basically a direct sequel to the first game, And also so far seems to have a fair amount of hype, what with that neat video of Dracula vs the armies of the brotherhood, and all that jazz. I feel that loS2 will do well. maybe not LoS well, but definitely better than MoF. MoF just had way too much working against it.