The column mass (ie, the spatial density multiplied by the length of the observer’s line of sight through the material) is highest at the limbs when looking at a gaseous shell. So shells like this look like rings from all angles. A related effect called limb darkening is the edges of the sun always look less bright, independent of viewing angle

The ring is not a product of the supernova. It's pre-existing material, that is being energized by impact of the supernova ejecta (it took a few months for it to start brightening).

There are actually three rings, a smaller one around the previous star's equator, and two larger ones above and below. The hourglass-like figure is actually quite common, produced by the star rotaton.

According to this article, this explosion is hourglass-shaped rather than spherical, due to the previous behavior of the star.

https://www.astronomy.com/science/why-does-supernova-1987a-l...

High speed rotation in an axis?