Peel brought in bmiBaby who then pulled out because they could make more money elsewhere, Peel brought in Globespan who went bust and nobody came in to pick up the pieces - the likely alternatives were established at LBA and NCL, who have now expanded to fill any gap.
Sure Peel know how to develop land - look what they've done at LPL, a better comparison for DTVA than Sheffield that people bang on about (underused airport, plenty of land, major rival just up the road), just at different points in the economic cycle.
Plenty of space at DTVA for development and Peel will probably make more if it is centred around an active airport, but where's the market for flights? The 900k in 2006 have now moved elsewhere and even at those levels the airlines weren't making money (or enough money!)