The gothic-themed exploration/action platformer is a niche. If you look at the number of individual backers and the average pledge (just short of 65k backers with an average pledge of about 85$) you will see that this genre is indeed fairly niche, but, and here's the big but, these niche customers are more than willing to pay a lot more than AAA retail price of 60$ for a game like that, which shows that they are a market that is a lot more willing to spend more money on it. You will see that most people who play the multiplayer Castlevania HD on XBLA/PS3 will have almost all, if not all, of the DLC for it, which all put together was sold for as much as the base game itself (15$).
However, as I've mentioned multiple times in here, publishers don't want some money, they want all of the money. They're not satisfied with a decently cheap-made game and have it sell for a few hundred thousand copies. They want a title that sells millions of copies. And Castlevania never sold millions of copies - however Konami had a small and willing base of customers that would buy most or all entries to the series.
Also, SotN is arguably one of the most popular title of the series, and I'd say it ranks after Castlevania 1 (NES) and Super Castlevania IV (SNES). Almost everybody who had a PS1 played SotN.
