Why does Preview remain active in the background?
This is by design and expected behavior if your memory pressure is green in activity monitor. Both iOS and macOS will keep some apps resident until compressed memory or paging is called for and then purge them. Changes in the kernel relating to App Nap, sudden termination, background processing allow many more states for applications than just running and quit. 

Why do you care whether the process is still in memory?  It's not running.  Preview.app supports Automatic Termination, the system will remove it from memory when it is necessary to to so.