If both variants are possible, what's the differtence in meaning? What emotional connotation does each of the two phrases bear?
1)You could have helped me. Why did you just sit and watch?
2)You could have helped me. Why were you just sitting and watching?
If I were a native speaker of English, I would never shut up. :-)
They mean the same to me.
They both bear the connotation of irritation or annoyance.