If I remember correctly that was originally left clear, along with Bankmore Square, and a strip all the way along Ormeau Ave and along behind the Gasworks because it was originally earmarked for the "Belfast Urban Motorway" (would have made a nice acronym

) back in the 60s.
As far as I know that project was killed off many many years ago, but I'm not sure why this land was never built on.
Edit: yup I do remember correctly:
http://www.wesleyjohnston.com/roads/belfasturbanmotorway.html
Bruce street needs developed, especially the south side which is all carpark