Answer: 
These communities were vulnerable because they were built on low-lying terrane of a barrier island. 
Explanation: 
- Barrier islands are 'coastal landforms' and a type of 'dune system' which are lumpy or flat areas of 'sand'.
- It is formed by tidal and wave action parallel to the 'mainland coast'. 
- The usual form of occurrence is chain format that consist of anything from a 'few islands' to more than a 'dozen'. 
- Hurricane sandy was the 'most destructive deadliest' as well as strongest hurricane of 2012 hurricane season. 
- It is the 'second most destructive hurricane' after hurricanes Harvey and Mariana in 2017. 
- The destruction was recorded to be about $70 billion. Minimum '233 people died' by sandy which spread in about eight countries. 
- Due to the location of the communities in venerable low-lying terrain of the barrier island, the destruction caused by the sandy hurricane was multiplied.