Indeed, many bigger VFX companies are on Linux. However, smaller shops are mainly powered by Windows (or at least that's the case here in Finland). Building your production pipeline on Linux is something that not everyone is capable of doing and in many cases not even worth the r&d costs.
I've tried for ages (since 2010...) to get the VFX studios to work with me on making Krita good enough for their needs. I've visited dneg, worked with Intel and other parties, but in the end, the studios yell they want to get rid of the shared windows/photoshop pc in the corner office -- but don't want to put in any actual effort. In the meantime, we're getting there anyway.
Foundry's Mari [1] is pretty heavily used in bigger VFX houses for texturing and it is also available for Linux. Likewise, Allegorithmic's (now owned by Adobe) Substance is available for Linux [2].
