what caused hurricane sandy to be a "superstorm"?

Hurricane Sandy was not actually a hurricane anymore when it hit New Jersey. Sandy merged with a cold front and cold air mass which made it extratropical. North East storms or Nor'Easters have their own rating scale that is different from hurricanes. This scale is called the Dolan-Davis scale and is numbered in categories 1 through 5. Cat 5 Nor'Easters are called superstorms. Sandy with a 941 millibar pressure and 20ft storm surge over multiple high tide cycles was what classified Sandy as a Cat 5 Nor'Easter on the Dolan-Davis scale. This is why Sandy was called a superstorm and not just a Cat 1 Hurricane.

I dont think Sandy was a "superstorm" in that sense, really it was a category 1 hurricane which is the smallest scale. However, because the Atlantic Ocean was unusally warm for this time of year it had time to grow in size to become one of the largest area-sized storms in history, and because it hit the US east coast it was an area that was extremely populated which lead to more damage.

Sandy dumped about 300mm of rain in a 2-3 day span, which while its a good amount it came down over a several day span. The issue was the area it hit was such a low lying region which is prone to flooding and storm surge, because of that a lot more damage was done then lets say if Sandy hit a lesser known area.

The news media's compulsive obsession of hyping anything and everything to satisfy their audience's short attention span.