There is no space in central (essentially the area bounded by the Circle line stretching down to Elephant and Castle). Lots of these developments happen in East London which has tons of brownfield land that used to be docks and industry. This is what happened to Canary Wharf and what they have been trying to do with North Greenwich and the Millenium Dome, and Newham and the Olympics. Stratford has loads of land and six train lines run through it (Central and Jubilee, North London, Great Eastern, DLR, Elizabeth) so it seems like the kind of place they would pick. Modern London isn't known for sustainable or sympathetic development. Go to Lewisham town centre and see how the new age tower blocks look completely out of kilter with the surrounding environment.

It's main purpose would be as an event venue where people would travel there deliberately rather than as a sightseeing item, and Stratford is a travel hub with connections to all other travel hubs in London and by extension the rest of the UK.