I don't think it's Nadia holding Janet back. I can't imagine someone at Marvel said, "Alright. We finally can have a Wasp book, but it's Janet or Nadia." And someone picked Nadia and everybody agreed there can never be another Wasp book.
Miles took over a popular character's book as opposed to getting a spin-off. His popularity is what earned him a spin-off. Janet's popular, but she's not Miles popular. That's what really holds her back. Marvel just doesn't think Janet can hold a solo when characters like the Thing can't hold a solo. Janet and Nadia's audiences are two different audiences.