This is because the first recipient of a favor may be tempted to cheat and refuse to pay it back when his turn comes. According to Darwin, delayed reciprocal altruism can only evolve in species that are capable of recognizing and remembering each other as individuals. Dawkins used a theoretical model to prove this. Imagine that there are Suckers, Cheaters and Grudgers. Suckers always do favors regardless of anything.
Cheaters never do any favors for anyone.
Grudgers will do favors for people until a cheater does not return a favor then the Grudger will hold a grudge against that particular Cheater. The Suckers would not last and die out very quickly. The Cheaters would last for awhile, but once the Grudgers hold grudges against all of them, they can not survive. The Grudger turns out to be an evolutionarily stable strategy against sucker and cheat, in the sense that, in a population consisting largely of grudgers, neither cheat nor sucker will invade. There is another category of subtle cheats who appear to be reciprocating, but who consistently pay back slightly less than they receive.