I would guess that the most relevant and meaningful information about a currency would be its purchasing power history.
Why doesn’t that info show up in WolframAlpha when I query “US Dollar”? Is the data not available?
The Wolfram|Alpha team is always interested in suggestions for new functionality and new data sources. You can let them know by submitting feedback:
For financial data, it's often hard to find a data source. And often those data sources don't allow data to be used in certain ways.