Why is WISE J0521+1025's distance so uncertain?
The distance is that reported by Bihain et al. (2013), which is based on a mean relationship between absolute magnitude and spectral type that has a lot of scatter.
i.e. in contrast to most (all?) the other objects in that list, there is no reported trigonometric parallax measurement for this very faint T7.5 brown dwarf.
In fact, if it turned out to be a binary brown dwarf, the distance could be incorrect!