What is the psychological explanation for the phenomena that a person sees the solution to a problem as “simple” once it has been found?
As @Arnon suggested in the comments, this looks to be a case of the hindsight bias, especially given the reference to "hindsight is 20/20" in a comment.

Via Wikipedia:


  Hindsight bias, also known as the knew-it-all-along phenomenon[1] or creeping determinism,[2] refers to the common tendency for people to perceive events that have already occurred as having been more predictable than they actually were before the events took place.[3][4] As a result, people often believe, after an event has occurred, that they would have predicted, or perhaps even would have known with a high degree of certainty, what the outcome of the event would have been, before the event occurred.