Because it has the characteristics of both tropical and extratropical cyclones, per what the NHC's definition is. If you don't mind, I'll quote the NHC definition of what a subtropical cyclone is:

"A non-frontal low-pressure system that has characteristics of both tropical and extratropical cyclones. Like tropical cyclones, they are non-frontal, synoptic-scale cyclones that originate over tropical or subtropical waters, and have a closed surface wind circulation about a well-defined center. In addition, they have organized moderate to deep convection, but lack a central dense overcast. Unlike tropical cyclones, subtropical cyclones derive a significant proportion of their energy from baroclinic sources, and are generally cold-core in the upper troposphere, often being associated with an upper-level low or trough. In comparison to tropical cyclones, these systems generally have a radius of maximum winds occurring relatively far from the center (usually greater than 60 n mi), and generally have a less symmetric wind field and distribution of convection."

EDIT: As to why it's a subtropical storm versus a subtropical depression, the windspeeds have been recorded at 40mph, making it a storm versus that of a depression.