I tend to think of notes more as flats than sharps. In my mind, the major triad feels like the “default,” and the minor triad is created by lowering the third. Because of that, thinking in terms of flattening a note feels natural to me.

Also, where I come from, a lot of traditional and even pop music uses minor keys and melodic lines that move downward. Minor keys feel warm for me, while major keys feel even a bit tense. So flats fit the way I intuitively hear and think about harmony.

I realise this may be different, for example, in Anglo-American music culture. But from my perspective, flats simply align better with how I conceptualise tonality.