Daisy was put in as an echo fighter because she was so similar to Peach in terms of her model to begin with. Hell, Daisy started off as a palette swap of Peach. Daisy was obviously put in before Waluigi because she'd take less time to implement and wouldn't need a new move set.

Waluigi would not work as a Luigi clone meanwhile since he has a different body type/structure and height from regular Luigi, and thus would need an entirely original move set and thus take more time to complete/put in. It wouldn't work as well as Male and Female Wii Fit Trainer or Peach and Daisy.

Just saying, Waluigi is different from Luigi the same way Wario is different from Mario.

Since Sakurai likely had less room for characters this time around and wanted remaining slots to go to ones from different franchises to join in; it was likely between Waluigi and Daisy, so Daisy was the obvious pick because she'd take a lot less time.

I remember when I used to be annoyed by "Ridley is too big" memes when it was popular.
Now this meme takes the spot.