That was basically because you could die from pretending to do it. The challenge was to eat a laundry pod. That's really obviously not safe, but biting a laundry pod and spitting it out after pretending to swallow and die for the camera seemed like a reasonable way to freak people out while skipping the dangerous part to a handful of teenagers. The biting step was the real dangerous one, though, as concentrated laundry detergent can corrode tongues and throats and windpipes really quickly, and you'd lose the capacity to decide what to swallow, what to inhale, and what to hold in your mouth and spit out within seconds. This kills the teenager. The news generally reported this as Teenager dies attempting Tide Pod Challenge instead of Teenager dies attempting to fake Tide Pod Challenge, which didn't tell teenagers it wasn't safe to pretend to do, but did make pretending to do it seem like a better prank, so overall only made it more tempting.

It is on the World Health Organization's List of Essential Medicines