What: Electric vehicle.
Why: Because there might be a slightly conductive fault inside the HV DC battery pack, think about a coolant leak.
Now, whether 6mA was a good choice is up to debate but it is what it is. You can argue lower current limit is always better for safety, as long as it can be implemented, and as long as it doesn't cause nuisance tripping on non-faulty equipment. 6mA DC leakage detection can be implemented for example using fluxgate sensors, so that's no problem.
In Europe mostly, AC leakage detectors ended up using higher thresholds (such as 30mA) because on AC, capacitive reactance exists and it causes nuisance tripping, for example due to EMI filters. On DC, such element does not exist by definition, so 6mA DC leakage has to be all resistive by definition, and on a 600V battery pack that means 3.6W is dissipated somewhere, which is becoming a fire risk. So using such low threshold is IMHO a good idea for safety. And you can see the required delay for 6mA is very long, don't remember exactly but it's some seconds, clearly suggesting fire safety is the real motivator. For protection against electric shock, the reaction time needs to get faster at some 30mA and beyond.