Sort of a follow-up to Toblet but
Nintendo just shoves Rosalina into every game...
I mean, at first she had a personality, now she's just like every other character...
Daisy clause invoked. RIP Daisy since she too had a personality but now acts as the 'filler' female sports character (which is really sad) in literally every spinoff ever (well not Puzzle and Dragons which is perplexing)
If we consider Roslina's spinoff appearances:
Mario Kart Wii onwards - probably the only true popularity wave that Rosie's had in spinoffs until the last year or two
MG:WT - Probably her first non-Kart spinoff, and even then, she's DLC. Probably Ninty's way to test the waters for Rosie's popularity in spinoffs outside of Kart, and I think it paid off.
Smash - well Rosie's from a pretty defining Mario title and her character contributes a lot to her popularity. Sure she didn't have the wave of spinoffs that Daisy has enjoyed (or rather hated, since this is the only thing keeping Daisy relevant now), but I'm led to believe (please quote something if otherwise) that the impact of Rosie in SMG and SMG2 >>>>> Daisy in Super Mario Land (and maybe MP3 as well)
MP10 - well, she cameoed in MP:IT, it'd be logical to elevate her. Also roster gender balancing never hurt.
Remember she's not in MTO.
P+D:SMB - this honestly came as a surprise to me as opposed to Daisy, but given SM3DW, pairing Rosie and Peach as playable ladies in adventure titles is an easier mechanical distinction than Peach with Daisy (Daisy is a faster/stronger Peach who can't float? Isn't that Toad?). ofc idk how P+D works but I'm guessing Peach is Fire and Rosie is Ice?
Character choice is a mix of introducing characters relevant to the franchise at that moment in time, as well as generally maintaining the previous rosters (not according to the later MP games - screw you ND Cube).
Bowser Jr has had waxes and wanes in relevance, ironically complementing the Koopalings' (until Smash I suppose, tho the NSMB titles from Wii onward have them together). Nabbit is currently relevant but I anticipate he won't be in a few years thanks to being an otherwise non-important enemy/villainous character (sure, he's been playable in a main platformer, but still, it's not like Nabbit is a driving force in the Koopa Troop). Diddy hasn't been super relevant in Mario following the GCN days.
Compare this to Daisy. Ignoring spinoffs, where is she relevant in the series?