Just to clarify before I go on my rant, I'm a Fire Emblem fan over a Smash fan.

There's a few problems and it's not just to do with Byleth.

1) A lot of FE characters are heavily meant to resemble Marth. Look at Pokemon, They also have 8 reps but there's a lot more variety in who was chosen.

2) 5/9 of these characters are clones. Or at least were, I don't know how much they've changed but that stigma of being a clone will never leave them. And clones are seen as lazy.

3) The swords; Even if Corrin transforms into a dragon or Robin uses spells they're still primarily sword fighters. You aren't just spamming your specials. You spend a lot of time with your normals which means you'll be spending a lot of it with the sword. Add that on top of Link, Toon Link, Young Link, Shulk, Cloud. Remember there are 8 FE reps now. That means no Crash Bandicoot rep, or Spyro, or Kingdom Hearts, or Halo, or Doom, etc etc etc.

Now keep in mind even if you disagree with one or all points, this is how the Smash Community perceives it.