"Every time". You mean like in Brawl? Where SV and FD were the best stages by a mile and a half for a lot of the cast?
On what metric? Compared to what? I have no idea what you're basing any of this on. Characters get advantage on FD compared to what? Characters get advantage on BF compared to what? I have this sneaking suspicion that you're unconsciously comparing it to Smashville...
No, I mean like every other time someone mentions something like that?
If you don't see the differences for these stages (with me even giving one example for Delfino and Halberd) you should probably check out the game a little more to learn about it.
Okay let me help you there... when I compare stages I compare them to neutral stages.
FD/Omegas, Smashville and Town and City seem like the most normal ones.
Why? Solid main platform, (almost) no interferance, nothing really stage-specific happening there.
Now Lylat can almost join them. It has a somewhat big difference to the others in swaying. It also has it's slopy and uneven ground, with pretty low platforms.
Now what are these differences of BF, Delfino and Halberd I'm talking about?
Well, let's start with BF:
The stage is very different from the other stages, and in my opinion in a bad way.
The blastzones are further away which means you're living way longer here than normally. Which shouldn't be a bad thing, it just messes a little bit with the players perception of the usual smaller blastzones.
Now the problem I have with BF is this: The platorm formation makes it very difficult to land if you're getting juggled. Someone controlling the inner stage has a very big advantage sometimes. This advantage is so big, and the jugglinghelp in the means of the platforms make it a really unlikeful stage in my eyes. I have this experiences on no other stage. In Brawl there were partially problems like this present. "camping below the platform while spamming stuff". It just makes approaching more difficult. Now if you're at the edge of BF or even at the ledge, or just above the platforms trying to land - it is way more difficult to get on the main stage safely than it should be.
(for the record: Dreamland 64 is my favorite stage in both Smash64 and Melee - it didn't have these problems)
Delfino is a pretty nice stage. It was already nice in Brawl. It's very different than almost all the other allowed ones again, though. There are walkoffs (which are a little flawed, but it's "okay" since it's only for short periods of time), water sections (very different from all other normal gameplay other stages have), and some layouts could be questionable.
The big thing here, though, are the very, very, very, very, very tiny blastzones during transformations, which basically "break" the stage a tiny bit. It's still pretty decent, but this is one flaw it has. It can net "random" KOs and players obviously try to abuse this.
Halberd would normally be a pretty nice stage, but the low ceiling kinda destroys it again. Dying from Rosalinas doubleuair shouldn't be news to anyone. Mario apparently has a few combos like dthrow uair uair upB that can instantly KO, and let's not forget ZSS, who gets superearly kills with upB again. Or just dthrow uair -> read then uair or upB.
You can basically die super early on these stages (making them able to give out very varying results, depending on if the superearly kills happen or not), while BFs advantageous positions are too skewed into ones favor.
There's always a very good reason for why I don't like some stages (and I know for it's like that for many others as well). You know I hated Lylat before, because of it's "random" (not really, but almost) "making characters airborne to interrupt any action they're executing". The ledge was a very small turnoff that I didn't mind too much in Smash4 (in Brawl it was very annoying for many characters, but in Smash4 many more characters should be fine with it), but now even that got turned around. It's now one of my favorites.
I (would) really like Delfino as a stage. The layout is varying, it looks good, it has some interesting elements (jump-through main platform, water, etc.), it's mostly fun to play on. But there is just that one problem which is really turning me off. The possibility of dying too early just because my opponent got a deciding hit at the right moment of the stagetransition is pretty scary when you're trying to bring consistent results. In Brawl I liked it more, although it still had problems in the form of MK sharking being a little op and chaingrabs on walkoff sections without any platforms to be safe.
I never really had a problem with Halberd in Brawl, but now that we have so many true combos and "op" moves like Rosalinas uair it seems like there is a little problem now...
I (playing Shulk), absolutely hate Smashville and FD and they definitely aren't good. Also there can't be only 2 characters that benefit from FD. In a matchup, one character is almost always going to benefit from a stage more than another character.
It doesn't matter what character you play. You probably hate SV and FD because you just don't like them (any reason for that?). I like playing on them with every character. They aren't that character dependant, they're just really balanced stages without too much interfering which is what competitive players should want.
Why do people choose these stages? Because they're their best stages?
Why are they their best stages? Maybe because they're most consistent there? Why are they the most consistent there? Maybe because there isn't any weird or "a little op" stuff happening on them?
Many players just try to have "no BS" happening to them so they choose the stages that aren't having any of the stuff that can bring inconsistancy easily, like superearly kills, missing the ledge because of Lylat angling in the other direction, etc.
That's why we see Smashville the most. I'm guessing overall Town and City could be the second most seen stage, since some people think they're at too big of a disadvantage on/or just don't want FD/Omegas. Of course some people just try to get the most possible advantage from CPing a stage so they choose Halberd if their character can KO easily off the top etc., so we see that quite often as well.