She's in her pop era right now. She'll go back to rap starting second era and once she starts doing more features methinks.
I can see Kanye having her do a verse on his new album tbh.
Plus she's (essentially) Grand Hustle First Lady, which isn't really aligned with indie labels or YMCMB, so she has few options outside of mainstream rappers on Grand Hustle or GOOD Music.
To answer the OP, I think its cause she really wouldn't add anything to the song, so they dont wanna work with her. She is also very hated by the urban gp, even if she us getting radio play there
Why does it matter, does that make her a better artist, no. Iggy can get hits on her own as a lead artist, not like some other desperate bitches who are on a million features a year, desperate for hits.