Tags work best that are not random events or sightings, but rather things fairly commonplace around the world that can be searched for and ridden to get. Not everyone has road kill or iron mines, for example. I located a mine somewhat near me, not iron, but lignite, but didn't feel like riding 300 miles just to get the tag. I'm sure others have mines closer, but we are barely into spring and lots of US riders are still in snow and poor weather at this point in the year, reducing that community's ability to participate.
As weather improves, more people will participate.