MongoDB secondary node does not become primary when other nodes are stopped
This is by design. If you want to run with only one node, restart that node without replica set switch. 


  If a majority of the replica set is inaccessible or unavailable to the
  current primary, the primary will step down and become a secondary.
  The replica set cannot accept writes after this occurs, but remaining
  members can continue to serve read queries if such queries are
  configured to run on secondaries.


Read these resources for more details:


Replica Set Elections
mongo replica set to single server
Voting in MongoDB