Why do chord progression generators only work for some scales?

Both ChilloutEngine and HouseEngine contain different chord progression generators. When you use a generator, you can activate the "Lock scale (if generator is compatible)" option. Click on the "More" button in the chords view to find it.

When "Lock scale" is active, you can set the "Associated scale" to a specific scale (e.g. "D Minor"). Most generators work fine for Major and Minor, but if you work with other scales - like Phrygian, Dorian, etc. - the Engine will ignore the "Associated scale" and generate another chord progression in Minor or Major instead.

Why does this happen? And why can't the generators "just work" for other scales/modes as well?

When you write chord progressions on a specific scale, not all notes of that scale are equally important. Some notes will be used way more often. Especially when you need them for the bass. And there are certain typical progressions as well: When you play note A in the bass, then it's more likely that note B will come next - and not note C.

As Major and Minor are so popular, it was possible to create a big database of common bass/root note progressions for these scales. The Engines will use this database to create bass note movements that make sense.

After this step, the Engine will search for chords that work well with the chosen bass notes. Depending on the generator algorithm, certain chord types will be used instead of others. A "Jazzy" algorithm will prefer 7th chords, a "Dramatic" algorithm will watch out for Sus chords, etc. Some generators might also use certain other tricks like automatic inversions, pedal notes, etc.

The point is: Without a good bass note movement, all the other steps won't be helpful either. If you use the "wrong" bass notes, the final result will sound off. Or it won't be typical for the chosen scale.

At the moment, I don't have enough datapoints to support all the other scales as well. I played around with different algorithms, but so far, I wasn't satisfied with the results. And this is why in most cases only Major and Minor are supported.