The oath of omerta (silence) is a part of the formal initiation ceremony of La Cosa Nostra, where a gangster is forever after considered a made man by his peers. Mobsters used to take this oath very seriously, but their loyalty was severely weakened when the government started sentencing narcotics traffickers to longer and longer stints in federal penitentiaries.
Carlo Gambino, a powerful New York mafia godfather, knew the only way he could keep the five families intact was to keep the mob out of drugs, or else every mobster who got arrested on serious drug-related charges would betray his associates in exchange for a lighter sentence. Gambino made dealing drugs punishable by death, hoping that his edict would keep the mobsters from ever getting themselves into a position where they'd be ratting out their friends to law enforcement officers.
After stiffer sentencing for drugs, the RICO Act broadened the list of crimes for which arrested mobsters could be tried, so more defendants were breaking the oath of omerta to lessen their substantial punishment. Over the years, some made guys broke the omerta oath just so they could sell their stories to the media, a tempting option that didn't exist back when the oath was first instituted.