Well there are several reasons, but this is the coolest: https://en.wikipedia.org/wiki/Island_of_stability

Basically, current theory suggests that once they get to 120 protons or so, they'll start finding stable isotopes again (ones that don't decay for minutes or more). A handle on a door affords pulling, and a hypothesis affords testing, and all that.

Edit: I should add that 115 might actually be within the island, but you'd need far more neutrons, which is currently not feasible.