[pdonis]

> why isn't the simple explanation that light loses energy over cosmological distances not also a valid explanation of redshift?

It depends on what you mean by "light loses energy over cosmological distances". See below.

> We already know that energy isn't conserved at those scales.

This suggests that what you mean by "light loses energy over cosmological distances" is that the universe is expanding while the light travels, and in an expanding universe, total energy is not conserved. That is true, and that is the redshift due to the universe expanding--it's not something different.

If, however, you are working from the hypothesis that the universe is not expanding, then energy is conserved even over cosmological distances, so I don't understand what you mean by invoking "energy isn't conserved" as an explanation for the redshift.

If you're referring to the "tired light" hypothesis, that's been known to be unviable for decades.