Why does Sigma E-mount 24-70 f2.8 cost so much less than the Nikon 24-70 f2.8?

In my area the Sigma 24-70 f2.8 for Sony costs:

• $1,600 CAD

The Nikon 24-70 f2.8 costs:

• $3,100 CAD

Why is there such a price difference?

I’ve always considered the 24-70 f2.8 lenses one of my dream lenses. With the Sigma and Sony mount, it seems very achievable. So much so, I’m thinking maybe I should get a Sony camera instead of a Nikon.

Is there a reason for this difference?