Why/when is AC-DC-AC conversion superior to direct AC-AC conversion?
There is a type of converter which can do this: the matrix converter.

In theory it can take many phases in and produce many phases out at quite a wide range of frequencies. It also has the additional benefit of not needing any power passives (in theory), or no large capacitor, no large inductors.

However, there are two golden rules with matrix converters


Thou shalt not short circuit the supply
Thou shalt not open-circuit the load


It is point #2 that makes the topology impractical as a simple loss of power will cause the inverter to blow up.

There is a variant of the matrix converter called the cycloconverter which uses thyristors and does not suffer the same issues as a full matrix converter. It, however, has a limitation of only being able to synthesise an output frequency around 1/10th of the input frequency. This limitation is fine for marine which typically uses 400Hz electrical supplies so generating 40Hz isn't too limiting for propulsion

So why AC-DC-AC instead of direct AC-AC ... The complications and limitations. A six switch inverter is extremely versatile.