What has made them popular lately? (at least with Chard).
I think they are only popular with Chard. I don't know why other developers haven't embraced the rooftop? Initially I thought it might be adding to the complexity of the building membrane and potential problems down the road but the inspectors I've talked to have noted that having a paving stone rooftop actually protects the roof membrane a bit more from the elements such as the sun?
There are various issues that can come up with roof tops that I've seen over the years. For example, the design at the 834 was flawed in that when you walked to the corners of the rooftop you could look into the penthouse living rooms so the developer had to install gates (seen in the left and right side of the picture below) to prevent people from walking to the corner in order to improve penthouse privacy.
However, Chard has improved every single roof top in his subsequent buildings. For example, the Juliet and 834 the elevator takes you to the penthouse level and then you have to walk up a staircase to the rooftop. Two problems with this, those with disabilities can't get to the rooftop and lack of penthouse privacy. The Duet, for example, the elevator goes straight to the rooftop.