Nationstar is not being left behind. Nationstar does business under the name of Mr. Cooper.
Nationstar (dba Mr. Cooper) is a wholly owned sub of Mr. Cooper Group (our shares are issued by Mr. Cooper Group) , which will be merging with Rocket Mortgage. In the merger, Rocket will acquire Mr. Cooper Group, including its wholly owned sub, Nationstar.
Any person who thinks otherwise has simply not looked at the readily available documents (and therefore any other opinions of such a research-impaired person should be ignored).