Because they don't.Why does everyone seem to think that snakeheads dont get along with both other snakeheads and other fish. It depends on the individual fish.
And I kept a great many deal of sneakheads, all of which have shown the exact same pattern, except for pleuros and maruloides. Channas either kill or let themselves die. They are exceptions, but it remains the rule. Fact of the matter is, your pulchra is not showing his natural behavior, and with an arowana? It's not going to last very long i'm afraid.I keep a pulchra with polys , catfish, uaru and an aro and have never had any problems . So say what you will, ill hold by what ive experienced. Personality does matter. Most of the time.
This is an argument that has been had back and worth over all many of species, I personally have a pleuro in my comm tank with my fahaka and CA cichlids.... however I accept that this is the exception rather than the rule and great care needs to be take if it's tried.