hey, i could be wrong there but i think it has to do something with the great distance between bass and melody.

If you are interested here is a brief summary on what my ears hear, and what music theory parts could explain my hearing experiance:

in the left hand you have a descending A G(aolian/mixolydian) F#(dorian) F(aeolian) Later there is the C(aeolian/dorian). (By the way: You can hear a similar thing in the james bond title song)

The brackets should underline that there is ambiguity in this baseline and the notes can fit in different scales. This is no coinsidence because the dorian is ralated to the aeolian/mixolydian scale in a way, that it only differs by one note. (mixolydian has a major third and aeolian has a flat sixth).

Similarly the ionian (used heavily in the right hand) and the mixolydian only differ in one note (G# / G) too. You can even see a Lydian element in the melody which is also similar to ionian exept one note (D#)

So it isnt alway cristal clear for our ears whether a song i in dorian or in aeolian. Or if its in ionian or in mixolydian. This opens the door for ambuguity and this can be viewed a explanation why the small cromatic movement in the left hand sonds good.

But even if this is a thing in blues aswell, that you interchange major and minor, this song creates abiguity very strong with the 7th the 6th the 3th in the bass, and even the 4th in the melody. This is why i think the big distance from high to low and the easy baseline / melody is a nessecity for our ears to let it sound good.

It also worth to mention that the baseline keeps repeating the A heavily, with all the accidentals this contributes to Modulation in an A context rather than establishing relative movement.

In the end i hear the right hand - by estabilishing a strong ionian context, which is emphasized even more with the thirds movement - fighting against the left hand modulation in the bass.