re: Why indeed?
I would have thought that they would have declared this in the Press release, but they don't.
The object is a candidate planet because it is only one-seven-hundredth the brightness of the brown dwarf (at the longer-than-Hubble wavelengths observed with the VLT) and glimmers at barely 1800 degrees Fahrenheit, which is cooler than a light bulb filament.
I'm not sure that I would call this thing a "planet". It's glowing pretty hot with it's own internal heat, it's further from it's Primary than is Pluto from Sol. It must be a marginal Brown Dwarf with Deuterium fusion just trickling along, or a very young object still collapsing rapidly.
You certainly wouldn't want to walk on it, even if it has a surface...