There have been a lot more than one word in these last few posts...
Dark Nemesis' point is valid, though.
First we had CV1. Then CV2 tried to build on that by adding free-roaming and somem RPG elements. But the game was pretty bad, so they went back to what they knew worked: CV1. So they took CV1 and built on it by adding swappable characters, branching paths, and some new enemies, incorporating the countryside from CV2. Then they decided to build on that in various ways. Some added 8-directional whipping and whip spinning. Some added kicking. Some added spears. Each added more traps, more enemies. Each made use of newer systems' hardware. But in the end, each built off of CV1, the tried and true. Then we entered the 3D realm and Konami tried a different approach. Rather than building off of the original, they took it to 3D and it flopped. They tried again (if at first you don't succeed, try again rightly so). And that flopped. So then they went back to their roots, to the tried and true. They built off CV1. They decided to further build off CV2. They did away with stair climbing and made the Castle even more open-ended, but scrapped the countryside. It was a success. But Konami, it can be argued, forgot its roots and forgot what made it a success. Rather than continuing to build off of CV1, they decided to build off of SOTN, even though it could be argued that SOTN's success wasn't inherent in itself but a simple consequence of not being a 3D flop like its recent predecessors. So Konami built off SOTN but the series lost steam because they were building off the wrong thing. Then they decided, "If at first you don't succeed, try again on a better system." So they made the PS2 games. And kinda flopped. They went back to SOTN and remakes and such. Then XBox 360 came out and they tried again.
And arguably (long, page after page of posts arguably) it was a failure too.
People clamor for a good 3D Castlevania. Konami's given them 5 or 6 and people weren't satisfied. The fact of the matter is, as history has shown, CV1 is the foundation all Castlevania games need to be built off of. You can modify F(x)=Ax^4+Bx^3+C(Dx-E)^2-F all you want, but ultimately it's a function of x.