Because they are excellent, simple as that.

My favorite Castlevania of the games I've played is probably Order of Ecclesia. That game was just straight up awesome and the gameplay mechanics were very good. It's definitely has the Metroidvania vibe and some brilliantly laidout level designs.

True, but no franchise that's existed as long as Castlevania has been without it's bumbles.  Mario, Zelda, Mega Man and of course SONIC have all had their stinkers but remain beloved for the great games.

OT: Castlevania is beloved because it did Dark Souls before Dark Souls was Dark Souls - tough but fair gameplay.  For every problem in a Castlevania game, there was a solution to it in it's level, i.e the Bat boss in the first area of the original Castlevania is much eaiser with the axe sub weapon.  It has great level design for the most part and some of the most iconic music ever made in video games.  

Symohony of the Night is a very important reason as well, but it's ignorant to ignore the classic platformers.  

I remember watching a series "retrospective" on Metroid, many years ago, by Game Trailers. I think they effectively summarized why that series, and extension parts of Castlevania, were so successful. In short, it was pointed out how Metroid combined platforming elements from Mario, along with the tool-driven adventure that Zelda wielded. I think that Castlevania really uses that to their advantage too. Paves the way for many other innovations and gameplay flavours on top of that :)

Castlevania was good until it become a Metroid clone. Thats when i lost interests. Castlevania 4 is still my favourite by a mile. Shame they dont go back to its original formula.

They are pretty decent, some of them. The NES ones are adorable, the SNES one is pretty good, the MSX one is nice, the Simphony on PSone is very good.
The other ones are tired and not very inspired, just because the series kind ran its course, no other reason...

Perhaps this proves that fluid controls are not as important as an environment that reacts consistently.

What tends to drive me nuts in the classic games is unfair hit detection and cheap insta-deaths. Castlevania actually did have it's share of cheap insta-deaths, but there's usually a remedy with one of the items (e.g. pick up the clock powerup to get through the brutal pendulums at the end of Castlevania III). The hit detection is usually solid, not only in enemies, but in platforming.

If you were to look at the Angry Video Game Nerd, for example, one of the most common "BS" moments in his platforming videos are missing a jump when the character is clearly "safe". Castlevania has some ridiculous jumps, but you don't usually feel like you should have made it, you just feel like the Belmonts are crappy jumpers compared to Mario :)

As for SOTN / "Metroidvania" - I enjoy those newer games too, but I'd love to see a return to a more classic style.

Cause Symphony of the Night is amazing. Other Castlevania's have trouble comparing... but it is a good reason why the series is considered good. But yeah, SotN, amazing game. Man, I gotta beat it

I too loved Simon's Quest. SOTN is, of course, a masterpiece.

To everyone yelling Symphony of the Night:

I’m not saying it’s a good or bad game (since I’ve not played it) but Castlevania was considered a classic series before that game was released. You just might be to young to remember.