Spy balloons like the one we shot down have incredibly large radar cross sections; they just get deleted by radar algorithms because they move so slowly.
From the article, a good explanation for why recent incidents used missiles rather than the cannons (among other reasons):
“In 1998, a rogue weather balloon from Canada deployed to measure ozone levels accidentally drifted away. BBC reported at the time that the airship was the size of a 25-story building and was operating at an altitude between 27,000 and 37,000 feet, so admittedly much lower than China’s balloon is now.
The incursion prompted attempts from Canada, the United Kingdom, and the United States to shoot it down, with two Canadian Air Force CF-18 Hornet fighters firing more than 1,000 rounds of ammunition from their own Vulcan cannons into the balloon off the coast of Newfoundland. That balloon took days to come down, at one point even gaining altitude and drifting across Canada and then out over the Atlantic Ocean, which made for a problematic situation for regional air traffic.”
