My guess is that you're using a modern high-capacity SDHC/SDXC card, not a bog-standard 20 year old ≤2 GB SD card. On the Wii, each game included its own library to talk to the various hardware (SD slot, USB ports, etc.).
Excite Truck was a Wii launch title, and at that time, only standard SD cards were supported, so the library included in Excite Truck can still only read from a regular SD card, even if you're running it on an up-to-date Wii/U.
To use the feature, you'll need to track down an original SD card, from before they were updated to the ≤32 GB SDHC standard. If you don't have any 20-year-old SD cards laying around in your home, you'll probably need to try looking for old stock at online retailers, as nobody is manufacturing original SD cards any more as far as I know.
eBay is usually a pretty sketchy place to find SD cards because it's a standard that's easy to fake by having cards mis-report their capacity to seem larger than they are, but you'd probably do OK looking for tiny-capacity original SD cards on there. That's not the direction people generally lie about them.