Cool question Scissors99, the short answer is the chelate effect is more pronounced in the diNOsar ligand. I break it down a bit more in the post below, and have a video response at around 18 minutes in for my stream (midnightduck on twitch). It can really help to draw this out.
first lets go over why Co3+ does not lose its ligands before the reduction. Co3+ is "low spin", which means that all of the electrons are in the dxy, dxz, and dyz orbitals are much lower in energy than the dx2-y2 and dz2 Orbitals. This is what causes the Co3+ center to never give up its ligands. However when it gains just 1 electron the ligands become labile again and it can give and take ligands. When those ligands are labile, the en ligand is less likely to stay on (exchanging for solvent), because it has less bonds to the metal. The diNosar ligand has 6 bonds to the metal center whereas the en ligands have only 2.
Hope this helps!