They tend to be more "forgiving". They flex a little more instead of cracking, and if you bend a rim off-road, they are easier to straighten and you can retension the spokes to aid that process. Usually (not always) they use inner tubes instead of being tubeless in design. Again, if the rim bends at the bead area, that doesn't necessarily mean a flat, like it might with a tubeless set-up.
I'm sure there are other reasons I don't know of.
2007 ABS Wee-Strom "The Silver Surfer"
2003 Kawasaki Concours "Mabel, the Flying Kaw"