For a country to exist, it needs a military or an ally with one.
It's easy to have no military, as long as you don't mind being completely controlled by a country with one.
It's easy to have no military, as long as you don't mind being completely controlled by a country with one.
TL;DR: Costa Rica has no threatening armies around it, so they don't need one.
There's also a bit about Panama being chill (possibly because of all the business generated by the canal), and Nicaragua basically held in place by international law.