Rosalina was maybe afraid Bowser Jr. would find her, but I mean, Rosalina should have known that Bowser was on Earth, that he literally almost killed Peach, and that Bowser Jr. was looking for his father, as happened in the movie. He managed to get to Earth, but Peach wasn't there. This seems illogical to me. Besides, if it weren't for the Luma that distracted Rosalina, she would have won, so her fear of Bowser Jr. also seems illogical. Also, since when did Rosalina know about Bowser Jr. and his threat? She could have gone personally and confronted him by surprise, and everything would have been solved. Besides, Rosalina didn't even send a Luma to deliver a letter to Peach. And how did Rosalina expect Peach to... Upon discovering that Peach didn't even have the technology to go to space, and also because Rosalina didn't send her a Luma for that purpose