Because joules are a measure of energy, and amperes and volts are not.
Everybody's goin' SI.
Volts makes no sense - if the skin is dry, even a high voltage may not cause much current flow.
Amps makes no sense as 100mA for a microsecond is very different from 100mA for a second.
However, joules is a measure of the amount of energy given to the body.
Joules is a measure of energy.
Volts and amps don't make any sense in the context of what we're doing with defibrillation. Voltage depends on resistance and resistance changes from person to person and with subsequent defibrillations. Same thing with amps; that depends on the time the current is applied.
200J of energy is always 200J of energy though, regardless of how many amps or volts it takes to deliver that energy.