Looking at it from their POV, and I'm not speaking for them nor am I saying that this conversation went down when I was a Team Member (as it didn't), but my guess would be that sometimes people don't get banned because they break one rule, it's because they break several. You then need to explain each and every rule that was broken, which isn't always possible as the data isn't kept in a format that's easy to search, and justify each one: That takes time.
I still think a quick one-liner saying "Jeff was banned because of some bad trading practices and poor reactions to moderation requests" would be enough, but others may want more detail which, let's face it, would be a real sod to sort out.
I can't see any moral reason not to disclose the reason for a ban (it's not without precedent, think Greekman), so I'll assume that it's a time/effort thing. I'm not having a dig at the Team here, as it's not like I particularly tried to change things when I was a Mod and perhaps had the ability to chat behind the scenes, so I'm as guilty as anyone for the status quo.