These stories are popular because they tap into our childhood fears. As kids, we were often taught not to trust strangers, and the ice cream man is a stranger who comes around. The thought of him being something evil plays on that basic fear. Moreover, the idea of something that looks so normal on the outside (a regular ice cream truck) hiding something truly terrifying on the inside is a classic horror trope that has always intrigued people.
Scary Ice Cream Man Stories often involve mystery. The unknown aspects of who the ice cream man really is, what's inside his truck, and why he acts the way he does all add to the allure. For example, the idea that he might be something other than human, like a ghost or a monster in disguise, is very fascinating. Also, it's a bit of a shared cultural thing. People like to tell and retell these stories, passing them on to create a sense of community around the fear.
They're popular because they play on the idea of something that's usually associated with joy (ice cream) and turn it into something scary. It's a contrast that catches people's attention.