Poverty: Stealing was an easy answer for those struggling to afford to live.
Geography: Territories were large- there were plenty of place for gangs to hide out.
Conflict over resources: There was conflict between ranchers and homesteaders, big ranchers and small ones and the Indians.
Weak Justice System: Corruption meant governors and law officers were often bribed by criminal gangs,
Problematic Lawmen: There were few honest, reliable men prepared to take on the difficult, dangerous job of being a lawman. Many had been criminals themselves!
Fear and Intimidation: People were too scared of the powerful gangs to stand up to them and stop them.
Independent Attitudes: Men sorted out their own problems, often with violence.. Killing in self defence wasn't against the law.
Vigilantes: Captured criminals were often lynched, ruining the idea of justice and fair trials.