You have to think critically about this. The third of the six chord is the tonic note of the scale. You have to double the third of the six chord when it is after the dominant but when you have that six chord in first-inversion you make it very much harder to resolve the leading tone in a satisfactory manner(Because one of the notes that can resolve that leading tone is already used.).
When you keep the six chord in root position you open your voice leading to more possibilities which simply makes it better.
I guess if the Dominant chord is in first-inversion then you have no choice but to have the six chord in first-inversion but at that stage, I wonder if it would not simply be better to use the tonic chord.
As for cadences, they will typically be in root position. Triads become more unstable as the inversions go up. This is less so in four note chords but for triads, it is very much true.
That is why second inversion triads are only used in a very specific manner. For cadences you don't want unstable chords, you want stable chords to add to the feeling of finality.