The diagram shows a stationary wave at the time(t=0) when all the particles are at rest! Therefore its kinetic energy is zero and its potential energy is maximum. The same its true when the wave assume the other extreme position as shown by the faint line at t= T/2. In the middle between these two extreme line at t=T/4 . the particles have maximum ke except the nodes. Therefore the energy of the stationary wave has potential energy = 0 and kinetic energy is maximum. The stationary wave in the other times are partly ke and pe but the total energy remains constant. The energy stays within the stationary wave and just exchanges between ke and pe in time. It 's as though the energy is trapped within the stationary wave and not propagated like progressive wave. It's easier to remember that the p.e is max. by looking at the wave as if it is a string that is stretched to a maximum at the two extremes. When the string is straight (least stretched) the p.e=0 and k.e max.
