Do you know the reason why?

I am guessing a combination of tighter manufacturing tolerances, smaller sector sizes, and manufacturers over time cheaping out would contribute to decreased reliability of 3.5" disks. People also, as far as I remember, universally used sleeves for 5.25" disks, but almost never had cases for 3.5" disks.

On the other hand, 3.5" disks have higher magnetic coercivity, which makes them less susceptible to data loss from stray magnetic fields (e.g. nearby motors).

I don't think you needed cases for them though since they covered the magnetic disk with their own sliding door while on 5.25" disks they were exposed to the elements. I once spilled jelly all over a 5.25" disk and took it out of the sleeve, washed it off in the sink, and put it in a new sleeve and it still worked :)

HD floppies were much worse, both 1.44MB and 1.2MB. They are just much more susceptible to defects from wear and tear because of the shrunken sector size. DD 3.5" wasn't as bad.