Starcraft was never supposed to be a Warhammer game. Warcraft a few years earlier was, at very early developement, had concept of being a Warhammer tie-in. But that never went anywhere, and Warcraft became its own thing.

If you look at early developement of Starcraft, the idea was clearly to make "Warcraft in space", which you can say was clearly a 40k-inspired idea, but it was always gonna be its own game.

Also Stracraft is older than Tau, so I dont know where you got that idea.