Because 2/3 of the story didn't totally revolve around Robin and because Chrom wasn't pushed to the side as soon as that event happened.
K.Rool's a villain, main villain > Playable actually...you probably want some unimportant Metroid one-shot hunter rather than Ridley, don't you? Also, Rosalina has made a much bigger impact than Bowser Jr...she was like one of the main plot points of probably the best Mario game ever made, you want to compare that to a Bowser Scrappy Doo and a generic Mushroom Kingdom guard?
none of the story but the chapters where Robin was mind controlled revolve around Robin. Valdar didn't need Robin in this timeline, Grima already had a vessel (which was robin, but another Robin, ok). the sotry doesn't revlove around ANYONE, it revolves solely around the fire emblem.
gangrel wanted the Fire Emblem, Validar wanted the fire emblem, can considering walhart kept the Vert gemstone and safeguarded Tiki (which had azure), i'm sure he was planning on getting the other 3 gemstones and the actual FE as he went through the other continent.
And no, Chrom wasn't pushed to the side at any point. quick recap of the grima arc AKA spoilers:
1- Chrom wants the last gemstone, Sable, to safekeep the FE. meets with Validar; later he and Mind Controled!Robin have a brawl over the damn shield. Both guys are important. IIRC in this chapter the murderous Lucina event happens, everyone is important here in a way (because this event is meaningless overall, since nothing truly happened)
2- the shepherds want to enter the dragon table, Aversa and the deadlords block their path: nobody is noteworthy.
3- The premonition coming true, vs validar, both guys are important.
4- Chrom is trying to get exalted Falchion, and is the only important party in this chapter besides Naga, i guess.
5- Another Aversa chpater of un-noteworthiness.
endgame- Both characters are important, Robin more so.
Overall, the Walhart and Gangrel arcs revolved much more around Chrom than Robin (if we can even say the Walhart arc revolved around ANYONE besides say'ri...)
Not to mention Chrom's passive role as the Leader is a more important and noteworthy role than that of Robin's Tactician role
now, for the Mario reps: Bowser Jr is more important than you think, to the point of being the MAJOR VILLAIn (just like Ridley in his normal games) of Sunshine and the first NSMB (where he lead the koopa troop himself after bowser died ingame, AKA 90% of the damn game); which is a more noteworthy role than Rosalina had in both galaxy games, since she did basically nothing during most of G1, being only relevant at the ending and prologue... and was basically an extra in 2, doing nothing truly noteworthy.
and that's obviously not considering Bowser Jr did more stuff in the galaxy games than Rosalina. Sadly, Playable > Villain, but
If we are to believe Aonuma, 3DW should've had a lesser development cycle than Smash, and as such Sakurai decided on Rosalian being playable fore that decision was made for 3DW. and villain > story NPC. That's obviously ignoring that Jr also had a lot of roles in previous Mario games, specifically
NSMBW which outsold both Galaxy games TOGETHER. (why did my font shrink?!?!)
As for toad, i suppose being in almost all mairo games ever, being plaayble in 3, including one of the most succesful games ever: NSMBW, and having bigger roles than Roslaina in all non-galaxy games they were in together (especially 3dW) is meaningless.
He outclassed Jr too, which is why i don't want him in smash right now., though he is a better choice than Rosalina. As for your metroid-theme insult; I want Ridley. I wanted both him AND Dark Samus tho, but i'm fine with Ridley alone. Sylux and Rundas would be cool, though
undeserving