There's no way to discuss Castlevania: Judgment (titled Akumajo Dorakyura Jajjimento in Japan, translated as "Demon Castle Dracula Judgment") without at first acknowledging just how weird it is. In a series dominated by platformers (both hardcore classic-style as well as Metroidvania affairs), Judgment stands in a completely different genre, a one-on-one 3D arena fighting game. That's not to say the series couldn't do with an infusion of new ideas once in a while, just that this game feels like it came in from a different world altogether.
Everything about Judgment seems anachronistic to the series as a whole. While featuring characters from across the whole of the series (villains and heroes alike), each and every one of them has been drastically redesigned, given new costumes completely unlike what they wore in their previous games. Some characters play very different from their former incarnations while others are given strange tweaks to their back-stories to try and justify their inclusion in the game. It's hard to view Judgment as part of the series and not just some strange off-shoot gaiden game that exists in its own continuity. And yet, it's part of the official series.
From a different perspective it makes a bit more sense, though. Via it's own story, Judgment is about a competition of the best heroes and villains, all drawn to together to fight for the right to take on the lizard-king Galamoth and his Time Reaper, two villains concerned with eliminating Dracula and remaking time as they see fit. The story is, frankly, a complete mess but it does allow series director Koji "IGA" Igarashi a chance to take a victory lap through the while series and have fun playing with all the "toys" he was able to create (and, for some reason, Cornell, who had previous been removed from continuity).
As a victory lap the game kind of works. It's a grand mash-up of all things Castlevania, combining every idea the designers could think of into a Nintendo Wii fighting game. Presumably the assumption was that the game would be able to do for Castlevania as Super Smash Bros. had done for Nintendo's own franchises. And that idea, to craft an accessible fighting game for a new audience, is sound... if only it had come in a better game.
Setting the weirdness of the story and style, Castlevania: Judgment just isn't very much fun to play. The core fighting mechanics are slipshod at best, and the whole package feels poorly balanced. It's like the designers really liked the idea of a fighting game but had little knowledge of how to do it. The game treads somewhere between accessible fighting game, with quick pick-up-and-play controls, and complex fighting combos (which are the exact opposite of pick-up-and-play). It throws the design in this weird area where it's neither complex enough for fighting games fans, nor super accessible for the masses that, presumably, Konami and IGA were hoping to attract.
It seems like neither general audiences nor fans alike were interested in what Castlevania: Judgment was selling as the game is tracked as one of the worst performing titles in the whole series, moving only 160,000 units across its whole life. Those are okay numbers for a portable title (although even those numbers would give Konami pause on a hand-held release), but this was a Wii game, a console with a huge install base. It's hard, from that perspective, to view Judgment as anything other than a failure.
In the end, then, Castlevania: Judgment was neither the grand celebration of the series is was potentially setup to be, nor a audience spanning new take on the series. It seems to have alienated more people than it brought in, only giving Konami more fuel for their decision to eventually put the whole series on ice.