Well, I wasn't there when it was written in stone, but if I were to guess, it's because of bean assignability.
Right. I'm less concerned with a bean whose type is, say, List<?>: that seems problematic immediately.
The part I'm having trouble seeing—and it is definitely my problem, I'm sure, not the spec's—is something like List<Comparable<?>>. Which assignment scenario is made difficult or impossible if this were permitted? I'm sure there is a reason this is being prohibited; I am just slow. If it's the same case as List<?> then I'm missing why.
Best,
Laird