Why commodities rather than equities?

What they are testing

Whether you have a reason, and whether that reason survives one follow-up question.

The answer that works is specific to the physical nature of the business. Commodities are constrained by things that exist: a berth, a pipeline, a vessel, a harvest. Price is the outcome of a logistics problem rather than a view about a company.

The answer that fails is any version of "I find it fascinating". That is not a reason, and the follow-up question will expose it in one move.

Answers that lose the room

Volatility, on its own. Volatility is available in every asset class.
Anything that would apply equally to a rates desk.
Naming a documentary you watched.