Well first off, Rosalina's importance to her series, even if substantially inferior to the existing four Mario characters, is still a whole lot more than Bandana Dee's to Kirby. Not to mention she did have fairly strong levels of popularity in Japan.
Regardless, it's highly likely she wouldn't have been included if she wasn't part of the Mario series, which is so large and prominent enough it can afford to choose the next tier of candidates and still have them be relatively on par with more high-profile characters with other series. Yes, uniqueness was probably a factor in getting Rosalina included, but the series she belonged to also must've been a huge element as well.
Might as well throw my hat in the ring.
Personally, when people we're talking about her, I was against her inclusion. Why?
First was moveset potential. Well, more the lack of it.
There was nothing to go by, nothing (IMO at least). Even characters like Sonic, with his moveset, at least had something to go by. I couldn't see Rosalina as something more than a Peach semi-clone. There was nothing that could make her unique. Think of it, she barely had any appearances to begin with, and there are other characters that would've fit that better.
Another thing was representation, we already have the big 4, why have another? There's no reason to add a fifth. It'd be like adding more chocolate to a chocolate cake. What's the point?
The only character I saw was Dr. Mario. But that's mostly due to him being in Melee.
Finally, why? Why is she more important than other, more notable Mario characters.
She only a supporting role in one of the major games (before anyone brings up her playable appearance in SB3DW mind you this was all before that), not to mention a cameo in another. The only other thing she had was the spin-off games. But with that, Waluigi or Daisy would've been a shoe-in by now.
That was just so much against her, I'd just laugh it off as a hopeless dream.
Then came December. And her trailer appeared for the first time. I'd admit I didn't believe it.
Then I sat down and watched it. It was weird for me, a character who I though had nothing somehow got in.
And, while I'm still kinda "meh" about her inclusion, I at least respect her more than before.
So? What does this have to do with Bandanna Dee?
Simple, while many know I'm not much into him, I at least see he has a chance. Think about it.
He comes from a popular franchise, he has moveset potential, and something to set him apart from the others. Sure, he isn't represented much. But, then again, so didn't Rosalina.