Why do potential energy decrease as two oppositely charged particles approach each other?
Because there is an increase of attractive forces ...?
2 answers
It takes work to move them closer together, therefore the potential energy has increased. I do not know why you think it otherwise.
Two oppositely charged particles would accelerate as they approached one another, so their kinetic energy would increase. Since their kinetic energy would increase, and the total of kinetic energy and potential energy is constant, then the potential energy would have to decrease in lockstep with the increase in kinetic energy.