Sometimes they didn’t want to pay for the testing and don’t want to be liable (Probably cheap product) sometimes they didn’t want to spend the extra 0.05¢/item to apply the proper coating/dyes/machine resistant features (Cheap product)
The rest of the time, it’s truly because of “specialized” material, like wool.
If you’re looking to buy clothing, it’s best if you simply didn’t buy anything that is “Handwash Only” (Unless it’s something like wool).
If everybody checked and avoided buying “Handwash Only” clothing AND dishware, they would disappear off the market rather quickly (With the exception of special materials that truly can’t be made machine-safe)