In HDFS Data Blocks are distributed across local drives of all machines in a cluster. Whereas in NAS data is stored on dedicated hardware.
HDFS is designed to work with MapReduce System since computation are moved to data. NAS is not suitable for MapReduce since data is stored separately from the computations. In NAS, data is actually copied whenever required and then computations are done which would be difficult to deal if data is huge.
HDFS runs on a cluster of machines and provides redundancy using a replication which makes it highly available and reliable. But in NAS is a single machine, therefore, does not provide data redundancy and hence reliability cannot be guaranteed.