I believe there is a lot of internal politics and dynamics playing here.

If you are Demis Hassabis and want to lead this new initiative:

- Why would you want to report to the CEO of another subsidiary instead of spinning-off another entity within the Alphabet conglomerate?

- You can let the DeepMind team focus in their original areas of expertise.

- You will have a new (and $$$$) budget to hire computational biology PhDs

- With the new entity, you can fail a lot and not make Verily and Calico look bad (inside and out XYZ)

If you are Verily and/or Calico:

- You don't want a newcomer bring the exciting new projects and ruin your projects, budgets, initiatives

- [very speculative] You don't like Hassabis' progress (or is envy of)

- You think this new project will fail and don't this under your umbrella