Chrom is a bastardised ensemble of Ike and Roy.
Luigi is just an ensemble of Mario and Pikachu (and Dr. Mario too considering he has Mario's old down B).
By this logic, they should both be echoes.
Let's take a look at Luigi as a character for a second. I'll address each character mentioned along the way:
"Why isn't Luigi an echo of Dr. Mario?"
Because Luigi has always had that down B, even in Smash 64. Mario used to have that Down B too. Dr. Mario has nothing to do with it.
"Why isn't Luigi an echo of Pikachu?"
First, Chrom isn't an Echo of Roy and Ike, he's just an echo of Roy that pulls things from Ike.
Second, Green Missile and Skull Bash may be similar conceptually, but they're not the same.
Skull bash charges up and auto-releases on full charge, going far and hitting hard.
Green Missile charges up but can be cancelled by charging it long enough. It deals fire damage, doesn't have the same animation, and can misfire, which is something unique to Luigi alone.
Also, it's only one move. I don't think a character that shares a single move with another makes them Echoes.
"Why isn't Luigi an Echo of Mario?"
Two reasons.
1. Different moves.
Luigi has many moves that are completely different from Mario, and unique to him
alone. Forward Air, Down Air, Side Smash, Up Tilt, Down Tilt, Jab, and Dash Attack. Some of these differences have existed even since Smash 64.
As for his specials, those are sort of a Lucas/Falco situation. They're fundamentally the same, but are changed drastically in ways that basically make them completely different moves. Luigi's fireball doesn't adhere to gravity, his jump punch goes straight up and has completely different hitboxes, and he retains his original Down B whereas Mario's has changed. It doesn't matter if Dr. mario has a similar Down B, Luigi had his first, a whole game before Doc was even conceived.
Echo fighters' specials tend to have very minor changes, mostly in minor animation or knockback/hit mechanics.
2. Different attributes.
Luigi actually retains his differing attributes from the Mario games in Smash.
He jumps higher than Mario, and he has less traction. Interestingly, he also runs slower than Mario, despite the fact that in Mario games, he's supposed to be
faster than Mario. He's taller by a noticeable margin, and he is capable of crawling, whereas Mario is not. he also can't wall jump even though Mario can.
As far as we can tell, Echo fighters seem to perfectly match their bases' movement and physical attributes. Lucina may be shorter than Marth, but it's so negligible that it doesn't really matter.
All this is what makes Luigi his own character.
TL;DR; He's too unique to be an Echo Fighter, something that applies to all of our current clones and semi-clones.
Clones - as we know them - exist on a spectrum, it's not black and white like so many people think it is.
I know this might be a little... extensive, but I really like Luigi, so I decided to go all out.