Nike's #1 criteria will be what they think you can do in the future. This is a business for them. If you aren't running top times or contending for international titles, then they are going to move on.
Nike also considers (to some degree) the social media presence, popularity, etc...
Will has had up and down years. He had a great year a couple of years ago and then seemed to be on his way out, but won the Bowerman Mile, which I think helped save him slightly.
But I have not heard about him racing anytime recently (in anything super substantive or fast). Not indoors and I cannot recall any major fall races.
It's possible Nike did a cutback on him and reduced his contract by some percentage. Or they told him they were ending the relationship when his current contract is done. Who knows? Will will speak about it when he is ready. He's never been shy about speaking his mind, and to his credit, I think he is well spoken. To those who wish to bash him for "running his mouth" or whatever those quotes were above, Will is one of the only people out there with the courage to speak up.
Good luck Will! Crush the trials and make Nike regret this - just like Meb has done!