Fermi ParadoxHow come we've never been contacted by aliens? The universe is so big and old, that statistically, some civilization must have already developed the ability of star-travel.
Prime DirectiveNever interfere with younger civilizations (from star trek).
The best reason for setting the Prime DirectiveAs far as I know, any contact between an advanced civilization and a native one in human history has always been disastrous to the native civilization (see: Chronology of colonialism).
The prospected visitors to earth have probably experienced these types of contacts within their planet before achieving interstellar travel. Hence, came to the conclusion that contacting less advanced civilizations can only hurt them.
Therefore, they set a rule (a "prime directive") to never contact less advanced civilizations.