Because Hart and Nilsson proved in 1968 that (paraphrasing) no other graph algorithm using the same heuristic can visit a lesser number of nodes than A*. This a

Because Hart and Nilsson proved in 1968 that (paraphrasing) no other graph algorithm using the same heuristic can visit a lesser number of nodes than A*. This algorithm is using a specialised heuristic that only applies on constrained grids of this type (effectively graphs with equal edge weights and constant branching factor at each node). In short, JPS is much less general.