There are two things relevant to this design:

1. what you are looking at is basically a freezer; For certain effects you need a very very low temperature.

2. Gold in general is a very good material as it is very stable and doesn't react with a lot of stuff (it is not very corrosive). It also conducts electricity very well.

I would also argue, that what you see is normally the open version of it or it was opend to show the inside: https://www.information-age.com/ibm-commercial-quantum-compu... they are in tubes; Might even be vacuum.

Also heat goes up, cold stays down; might also have influecned the design but actually you are probably better of asking a physics person :D