From the NHC Description for a Subtropical cyclone: "A non-frontal low pressure system that has characteristics of both tropical and extratropical cyclones."

The key is a "non-frontal low pressure system". Even yesterday the system maintained a decayed frontal trough that extended well to the southwest of the low. If the system could have separated itself from the old frontal trough it would have satisfied the definition, but I don't think that it quite made it. Another factor was the lack of any true tropical banding that would have enhanced convection - even if that convection was well removed from the center. The general eastward motion was not anticipated - at least not to the extent that did occur - and that probably placed the system in position to force interaction with the frontal boundary to the east and prevented full hybrid transition in the lower levels of the system. As NHC noted in their new Special Tropical Disturbance Statement at 11am AST, there is still a chance for reclassification in the next 24 hours, but this is not a high probability if the system continues to maintain an eastward component in its movement. It would probably take some new data, i.e., a delayed buoy or ship report before post analysis would be considered - but even that could still happen.
ED