Why do diesel engines require a significantly higher compression ratio (typically 15:1 to 22:1) than petrol engines (typically 8:1 to 11:1)?

1. To allow the engine to spin at higher RPM.
2. To heat the compressed air to a high enough temperature to cause spontaneous fuel ignition.
3. To prevent fuel from leaking out of the exhaust valves.
4. To reduce the weight of the piston assemblies.