1. Peel are a property development company;
2. Local authorities, regional development companies etc like airports as they see them as key to economic development;
3. DTVA has lots of surplus land;
4. put these three together and you can develop the land attached to an airport 'easier' (planning permission, funding) than you could if it was elsewhere (look at what Stobart's are doing at Carlisle, look what's happened to the former Speke airport site at Liverpool);
5. If the airport makes a few bob from aviation, all well and good - if not you've got a site where a lot of the infrastructure you need to make it attractive for development has been paid for by somebody else (see point 2 and look what happened at Sheffield City).