Of the various Castlevania games that have been pitched and then cancelled by Konami (a remarkably small amount considering the long lifespan of the franchise), Castlevania Resurrection probably hurts fans the most. Considering how far along the game was, it's a bit of a surprise the game was cancelled straight out, and considering the buzz behind the game, more than a few fans were disappointed by its cancellation... not that it's entirely a surprise the game never came out.
In development for the Sega Dreamcast back in 1999-2000, Resurrection was hobbled at the outset by the console it was slated to be on. Much like the Sega Genesis 32X attachment that was to be the home for another cancelled game, Castlevania: The Bloodletting, the Dreamcast was a poor seller out of the gate, meaning that a number of game companies had to decide if developing original games (and not just ports) for that system made financial sense. There's a reason that the majority of games for also-ran systems tend to come from the company that put out the console -- most other companies don't have a financial stake in the success of the system so they have no reason to develop for it when there are better-performing systems on the market.
But the system it was to be released on wasn't the only reason the game was cancelled (even if it certainly served as final justification). Reportedly there was friction between the U.S. development team behind Resurrection and Konami's main headquarters in Japan. As the story goes, despite an initial agreement on the direction for the game -- a 2.5D platformer starring Sonia Belmont (of Castlevania: Legends fame) -- Konami was apparently unhappy with how the game was developing, potentially expecting something different than the demos they received. When it seemed like the game would not be able to shift to the (possibly new) direction the company wanted, they scrapped it altogether.
These kinds of decisions and direction changes from game companies aren't entirely unheard of; different bosses from different divisions expect different things, and when projects don't meet certain notes, or when new ideas are dreamed up independent of the product in development, whole portions of games have to be scrapped and redone. Diakatana is one famous example of this, with Duke Nukem Forever being an even more widely known example -- both were games that were overhyped during their time, took much longer to develop than expected (due to shifting ideas and new engines being implemented) with the final released product being worse than anyone could have even reasonably wanted.
For a flagship series like Castlevania, then, it's reasonable to assume Konami had one idea for how the game should work, and when Resurrection seemed beyond saving (for what they wanted, even if it was close to completion), killing it would have been the best option. Plus, consider that Sonia hadn't starred in much loved game before Resurrection (Legends is not a very good game) so there wasn't even a marquee character attached. Also, the last big Castlevania at the time was Symphony of the Night and Sonia's new adventure wasn't anything like that game (more platformer than Metroidvania).
Add all that to Resurrection being, effectively, a 3D game (and the series had stuggled with 3D -- see Castlevania 64) and it's no wonder Resurrection was canned. The stars, apparently, were aligned against this game.
Legacy of Resurrection:
Unlike with The Bloodletting for the 32X, Castlevania Resurrection has not had an upbeat legacy since it's cancellation. For starters, Sonia was soon removed as the originator of the Belmont clan (replaced by Leon Belmont in Lament of Innocence). She was then fully removed from continuity, with her one released entry, Legends removed from the official Castlevania timeline. Despite other games also being removed before being added back in, Sonia's adventure is still the one game Konami like to pretend never happened.
Also unlike The Bloodletting, the heroes of Resurrection never found new life in later fan-made materials. No one ever tried to make a new version (fan brew) of Resurrection, and Sonia hasn't shown in in much of the way of stories or comics since the cancellation of her second game. The other hero that was slated to show up in the game, Victor Belmont, gets even less love still, showing up nowhere else besides a few promotional materials for this cancelled game.
Even the game itself has disappeared almost entirely from the web. A few screenshots and a couple of trailers still exist online, but even a supposedly leaked ROM of the game has all but disappeared completely, a vapor or vaporware lost to the ages.
So it's seems that the great legacy of Castlevania Resurrection is how little impact it's had on the overall history of the series. Despite being nearly completed, despite an interesting premiss and a second appearance for the first female Belmont in th franchise, Resurrection is little more than a footnote now, a story to be told and forgotten for this long-running franchise.