Several reasons, but the Main one is :- They don't need to.
DmC was far more successful for Ninja theory than it was for Capcom. It showed they were a studio that could deliver the goods under immense pressure and They managed to inject alot of their own personality (and thus showcase their strengths) into a existing ip.
Now they can return to making fresh IP in their own trademark style, knowing at least to publishers, they have proven themselves that tiny bit more.
Make no mistake. DmC was Just a job for them. They don't have any passion for the series or even the rebooted ip. Its simply a showcase of what Ninja theory can do. They will only revisit that if they cannot get any other work.