[ggm]

This has chicken/egg qualities. Sloths may have poor eyesight because the energy burden and genetic expression behind better eyesight doesn't pay in their niche, but that could imply it's the downgrade option after they slow down, not the input condition which led to slow lifestyle.

Similar to why some small things become giants and some giant things become smalls, it all depends on the niche, and the input conditions before transformation through time and genes.