Assuming that Canon is not astro-modified, the answer is because a whole lot of the light you're trying to capture is being blocked. There is a filter in front of your sensor that blocks the deep reds, to which the Hydrogen-alpha emission belongs.
It's not that the nebulosity is "bleeding" into the dark space. It is that your camera is blocking a lot of the emitted light, so you have to stretch it considerably just to see anything, which in turn is exposing the underlying noise.
The most obvious noise component is as Mark just wrote in his response: walking noise.
Solution for the signal: a whole lot more total integration time, or modification of the camera to remove that filter, or purchase of a dedicated astrophotography camera.
Solution for the walking noise: see Mark's response
Edited by jonnybravo0311, 17 September 2021 - 09:40 AM.