I believe it's likely most were not wearing seatbelts and this contributed to the high death toll. It sounds like there were ejections as well.
20 killed in Schoharie limousine crash
Witnesses at the scene said several bodies could be seen on the ground outside the Apple Barrel.
It is possible it was just a horrible crash that was not survivable for any of the passengers.
However, I don't believe that is the case.
I believe that the conversion of the vehicle likely added to the death toll significantly.
Limo Crashes Into Parked SUV in Upstate NY, Killing 20
A grand jury found that vehicles converted into stretch limousines often don't have safety measures including side-impact airbags, reinforced rollover protection bars and accessible emergency exits. That grand jury called on New York Gov. Andrew Cuomo to assemble a task force on limousine safety.