Because we're basically looking at it from the "top" of the accretion disk. That's not exactly correct, but the way light and gravity mix the image isn't perfectly uniform.

The brightest edge of the picture is the matter heading toward us, where as the darker edge it's heading away.

The image you shared has a bit of artistic touch. It's hard to visualize how a disk of matter spinning in one plane can emit light in the massively warped space around a black hole.

Possible we're looking at the top and the band is a disk around it. Dunno for sure.

the cosmic goatse

it's a false color image

it's too tiny to take a picture of using a traditional telescope, so instead, they use multiple telescopes around the Earth, and piecemeal that data together. Which means they have to reconstruct the missing details (it's not made up, it's more like playing "connect the dots" with tons of math)

the final image is a composite of 3 different grayscale images, taken at different wavelengths of light.

The resulting black and white images are given different colors, then blended together (which is pretty similar to how cameras take images, they just map the grayscale images to colors we can see with our eyes)

I hate to break it to you, but every photograph you've ever seen is a simulated image.