Pixelmator could never compete with Adobe. Their expertise is on Mac and until now they didn't have the resources to make a big product like Photoshop or Illustrator (at some point they shared the idea of making a vector graphics product but it was abandoned).

Another point is macOS has a significant market share in the creative industries. Personally I know zero designers/illustrators using Windows. My hunch is Mac users represent probably 50% or more of Adobe users.