Maybe it's more BNFy in Destiel or Wincestland, I dunno. But in addition to just general popularity there was some real (and very weird) social capital that came with being a BNF in FMA fandom that is, in retrospect, just super super bizarre, and that I don't think has been at all mimicked in my experience of SPN fandom.
Mind you, I was never "in" with the FMA BNF crowd, but at the time I really wanted to be. I wasn't ever smart or cool or talented enough, LOL, whatever metric existed to make these distinctions. I was friends with one of them--just, outside the circle of their other BNF friends. XD
In any case, FMA is the first (and last?? maybe??) fandom where it was ever love at first sight for me. I remember finishing up the Johnny Depp Sleepy Hollow late at night and being home alone in an unfamiliar place (the house my dad was renting, after separating from my mom). Sleepy Hollow was not sleepy-making, so I went channel-flipping and ended up watching Episode 17 of Fullmetal Alchemist.
It's a great episode, a "take a moment" kind of episode that exists as a breath between the end of one story arc and the next. It's beautifully drawn (both literally and literarily), and does such wonderful character- and world-building. I absolutely love this episode, to this day, and I wish that every series I have ever loved had one. It's like if all of the best coda fic SPN has to offer existed in canon. We just watched it last night and UGH. <333 Love.
In any case, on that night in 2005, FMA became my fandom. In my mind this only goes through 2006/7, though according to my FFN account I was posting fic through 2008 (even though I KNOW that was Bleach/Naruto time). There's a lot of fic that never made it to FFN, as well, and exists purely on this journal, so deep I may well never find it, so I'm not sure how accurate those datestamps are to when I actually wrote the fics and posted them originally. I may have just been trying to archive them somewhere.
While I was deep in the throes of FMA fandom, in 2006, an old friend from elementary school emailed me about her newest obsession, a show about cute boys who fight monsters. And she linked me an episode and told me I should watch it. That show was Supernatural, and the episode was somehow also episode 17, aka "Hell House."
And I... didn't get it. XD I didn't get why these boys would be attractive or why their characters would be compelling or how I could have a connection to any of it. I didn't get any of it.
LET ME BE CLEAR, THIS IS NOT A KNOCK ON HELL HOUSE. I LOVE HELL HOUSE (or at least, I do now). But if you remove it from the context of the rest of SPN, I do think it's hard to know why any of it would be compelling. At the time it just felt too slapstick, too stereotypical of some version of sibling relationships that I didn't find relatable. (Present! me would like to note that obviously the Winchesters are great everything is great every single second of them is the best second. But tbh the pranks are probably only endearing if you're well aware that Sam and Dean have other ways of interacting with each other, and that these ways are more common to them. It also helps if you know their arc, who they are, and that 1x16 "Nightmare" was the episode that came right before this one.) So in spite of the fact that I was at that very moment literally in over my head with a fandom for a show starring trauma brothers--albeit deeply not-codependent, and unproblematically devoted brothers with a generally more solid attachment to their respective moral compasses--SPN just wasn't for me at the time.
The first season of FMA stays fairly small, before opening into a much more ambitious political/moral drama that is exceptionally well done and tightly composed and really not about any kind of emotional brother love fanservice, lol. Which I respect as a narrative 10000% but also probably love less emphatically, because at my core I am trauma brother trash and that's all I really want in life.
On returning to this story now, so far I'm feeling similarly to how I felt about Inuyasha--I'm enjoying it, and am in awe of just how good it still very much is, but I that ebullient, no reserve fannish love is behind me. I would say that I'm significantly more invested in and in love with Sam and Dean and the kind of people and brothers they are than I am Ed and Al of FMA, but I don't know that that's fair. It's not fair to compare a current fandom with a past one like that. Of course I love Sam and Dean more; that's my current interest. XD On the other hand, I've now been in love with SPN longer than all of my other fandoms combined, so there's that, hahaha. Wasn't ready for it in 2006, but apparently starting from 4x01 in 2009 did the trick.