In the case of the Queensferry Crossing I wonder if ease of cable replacement is one of the reasons behind the design choice. The Forth Road Bridge [0], a suspension bridge over the Forth to the East of the Queensferry crossing, has suffered from cable corrosion [1]. There is no easy way to replace the main cables in a suspension bridge without first removing the deck it suspends, causing years of service interruption. In a cable-stayed bridge it seems you could perhaps replace one cable at a time while the surrounding cables take the load.

I live with a view of the bridges, it has been fascinating watching them construct the new bridge.

[0] https://en.wikipedia.org/wiki/Forth_Road_Bridge

[1] http://www.bbc.co.uk/news/uk-scotland-edinburgh-east-fife-32...