Probably mostly economy. Although I bought Sigma 50-150 2.8 lenses for our Nikon and Fuji DX bodies, because no one was making anything with a similar range and aperture at the time. It’s now relatively old, and out of production, but still a rock-solid performer.
And that is how they started, offering stuff the 'big guys' did not. At first the quality was so-so 80/90s) but now I would be hard pressed to see any difference.