The explanation for Line 54 of Part IX of Form 5329 is in my CA return. Why can't I see it in my US return?

Because TurboTax is designed so that information flows from the federal return to the state return, not from the state to the federal. That's how all tax software is designed because all state income tax returns are based on the federal return. 

If you already have a well worded statement, you can simply type it into the space for a statement on Form 5329.