Why are telescope mirrors nearly flat?
The main reason is that the red mirror produces a smaller image in its image plane, which requires a smaller pixel spacing in the detector (or a stronger eyepiece) to get the same resolution.
In a Newtonian telescope the focal length cannot be too small:

(Image Krishnavedala)
Also, a more ideal shape is a paraboloid (not a sphere). Spherical mirrors are relatively easy to manufacture, and &quot;nearly flat&quot; ones are very close to paraboloids.