how was this not directed by ken russell?

I agree with you. In my mind for years I thought that Russell hd directed this. He had made a biopic for BBC about Isadora that was released at the same time, with an actress who actually looked a lot more like the real Isadora Duncan (heavier, less willowy) and Russell's pic came out about two weeks ahead of the release of this film. I think it's the British connection and the congruence of the release that led to the conflation, at least in my mind. If you check out the collection of Ken Russell's earlier biopics (Ken Russell at the BBC, which includes his treatment of Isadora) you'll see him interviewed and he mentions the confusion. Hope this helps.

This even has a screenplay by Melvyn Bragg who worked several times with Russell. Reisz's film is fine, but I think Russell would have brought more immediacy and imagination to it.

I'm fine with the film as it is.

Isadora's story needed to be firmly grounded in mundane reality so she'd have something to struggle against, and you weren't getting that out of Ken Russell at the time.