Why is powering on the APU so low on the A320 emergency checklist?
The assumption was that a multiple engine failure happens at altitude, for instance in cruise while flying through volcanic ashes. This happened to British Airways Flight 9 and to KLM flight 876, with all four engines switching off due to the ash cloud. Total engine failure at altitude buys time, and if time is available first priority is to attempt to re-light an engine. APU will probably be off during cruise, and given the choice to re-light an engine or to light the APU it is clearly the engine that is the most beneficial to start.

All-engine-failure in airliners is very very unlikely. It is a function of number of engines - in a four-engined aeroplane, the chance of all four engines failing is assumed to be super extremely incredibly unlikely. With a statistical failure rate of 10$^{-5}$ per flying hour per engine, a four-engined aeroplane would experience all engine failure due to statistical causes once in $\frac{10^{20}}{24}$ flying hours1: a billion times a billion times four. So very uncommon that the only practical consideration would be a common mode failure from external causes, such as the volcanic ash cloud.

For a twin-engine like the A320, chance of all engine failure due to random causes is 2 * 10$^{-10}$, still considered an Extremely Unlikely event. Procedures are written for the least unlikely events, and updated according to events that actually happened. Now that a dual engine failure during take-off has actually happened, updating the procedures will be the result of failure analyses: how likely would it be to happen again, or what other procedures can be put in place to avoid this situation.

 1: The chance that any of four engines fails is 4 * 10$^{-5}$ per flying hour. Then 3 * 10$^{-5}$ per flying hour for the remaining engines etc.