Why would you mention something while it was not relevant? The Weasley's dad was known to be intrigued by things of the Muggles, and was known to collect items such as the specific Ford (car not available in the US so the reference was a bit lost).

If Ron had told Harry about the car in some random conversation in a previous book, you would probably then complain about some bit of obtrusive foreshadowing ruining the plot line. Okay, maybe not you specifically, but that's another common trope of criticism of fiction.