I think in the UK sense 'tabled' is borne out of a pretty literal meaning - based on the proposal literally being brought to the table where the committee are meeting to assess (and potentially vote) on its viability.

Yeah, perhaps. But the table also has all kinds of other things on and around it. Random papers of varying importance, microphones, Representative Smith's porn phone with the privacy filter, and maybe a ham sandwich.

A table (if there even is one present at all) seems like about the least important part of a group discussion of an idea.

it's the item of furniture that everyone is sitting round. the fact that people might put other things on it is about as relevant as asking why one particular person is called the Chair when everyone is seated, or why it's a conference room when the cleaners happen to also do hoovering in there.