Though above all of the possible scenarios that lead to this, there’s one thing that overrides any of what we just said: Billie Eilish did it because she, in fact, wanted to.
So shifting to a full head of blonde hair was probably an easy switch for Eilish, as she was already part of the way there.
Part of the reason that Billie Eilish may have gone blonde, at least from the perspective of anyone that has dyed their hair before, is the fact that the green in her hair would have required a high level of lightening to achieve.