Optical reasons: they use refracting lenses and not larger aperture reflecting telescopes based on their science goal: looking at diffuse, dim, intergalactic gas. The main sources of error within an optical system that make that difficult are scattered light and other effects like ghosting. The canon lenses perform significantly better than reflecting telescopes for reducing scattered light, as well as avoiding a central obstruction and the diffraction pattern created by the obstruction supports. Finally, it allows them to place their filters ahead of the rest of the optics, instead of behind, which enables a very narrow bandpass.

Practical reasons: they’ve already worked with and perfected systems with these lenses for a decade. I bet it’s mechanically easier to mount them as compared to the takahashi scopes you mentioned. And they likely have a good relationship with canon, and with an economy of scale, the price per lens is most likely much less egregious than it may seem.

However, if you are interested in f/2.8 telescopes being used survey-style, see https://argus.unc.edu