Why is she so short?

You topped the plant.........this keeps them short and bushy! 18/6 is optimum, plants NEED adequate sleep.

The 20 hour light schedule is part of the culprit here. Plants do a lot of growing in the dark, and if this is a photo period plant, then she really likes her 6 hours sleep. I usually switch my photos over at 18 to 24 inches. That will give you a plant from 4 to 6 ft. usually, when she is done her stretch..

I wouldn't worry about her height... when you flip her into flower, she'll start stretching and I think you'll be pleased. The grow looks really, really good - I think you've done a great job! A different strain may have been taller but this is a gorgeous looking grow & I'd take that over height any day of the week! Good luck with the flip and the flower!