not to be that gurl but originally Phoenix was given to Jean to put her on the power levels of Ororo. There was an interview Claremont did about this years ago:
That said, I dont think the thor or scarlet witch are anywhere near the level of Phoenix. Phoenix to me is a multiversal threat (maybe just a universal), as the white Phoenix of the crown she had the power to impact galaxies. The same isnt true for Thor and Scarlet Witch. So I think it makes sense to have her be separate from the Phoenix to stand on her own power and see what she without the Phoenix can really do.