Why are flash ratings given in watt-seconds instead of joules?

Mostly, it’s convention. Watt-seconds and joules are the same unit of energy: 1 Ws = 1 J.

In flash photography, manufacturers traditionally use watt-seconds because it relates more directly to electrical power over time, which is how flash energy is stored and discharged. A flash capacitor stores electrical energy and releases it in a burst, so “watt-seconds” feels more connected to the device’s electrical behavior than “joules,” even though they are identical mathematically.

It also matches other common electrical-energy conventions, such as kilowatt-hours on utility bills rather than megajoules.

One important point: a flash’s Ws rating refers to electrical energy in the unit, not the actual light output. Different flashes can convert that stored energy into light with different efficiencies, so the same Ws rating does not always mean the same brightness.

So the short answer is: there’s no special technical reason that prevents using joules; the industry uses watt-seconds because it’s the established, electrically intuitive convention.