See here for information from an Adobe employee:
https://forums.adobe.com/thread/1812515
" We had to remove the thumbnail preview functionality from Acrobat
and Reader DC for technical reasons. We know there are users who value
it but certain design considerations forced us to make this change."
Karl Heinz Kremer
PDF Acrobatics Without a Net
PDF Software Development, Training and More...
http://www.khkonsulting.com