They resigned (or were fired) because the business no longer needs their unit, which puts a damper on their impact and usefulness. It also makes them a cost center in a business that is striving to become profitable.

That is the simplest explanation, it's a tale as old as time. And is fundamentally explained by a very plausible pivot from "World changing general purpose AI - believe me it's real" to "world changing LLM integration and innovation shop".