Rosalina & Luma can dominate and usurp the battle easily by using precision. Using the Luma is vital while fighting an opponent. Although you don't need the Luma, the Luma facilitates Rosalina substantially. Personally, I've grown to multi-task with the Luma, and I have won many battles while utilizing Luma. I have also learned to playing the same without the Luma. She may not be AS STRONG, she isn't completely punishable.
My hypothesis is either that they cannot properly use the Luma to help Rosalina
OR
They prefer to use other characters that they feel are better than Rosalina.
(Rosalina is the best in my opinion!)
Overall, like any other character, she has minuscule flaws, but her recovery, great defense, and overall power is what makes her awesome. Some people just prefer to use characters like Diddy Kong and Sheik, and they believe that they can win with those characters. Even though Rosalina is a top tiered character, that isn't the reason why I used her. To be honest, I don't really want a hoard of Rosalina users just because she's top tiered.
From the day SSB4 was the day that I knew that she would be my main!