Curate, connect, and discover
Reasons people may decline to comment or kudos on fanfiction they enjoyed:
They consider comments and kudos transactional compensation to the author, and they don't feel that the work was good enough to deserve compensation, or they don't believe they owe compensation to fan authors in general.
They consider comments and kudos as feedback, and they don't feel that the author wants or needs feedback from them.
However, comments and kudos aren't compensation, and they aren't even necessarily feedback either.
Comments and kudos for fan authors are:
encouragement to keep writing
reciprocation of a bid for connection
interaction with your community
confirmation that they're not the only one interested in their story
Please don't consider comments, or especially kudos, purely transactional or purely a form of feedback. Comments and kudos are a way to show authors in your community that you're glad they're there.
Authors aren't "entitled" to kudos or comments, but readers aren't "entitled" to fic, either.
Fanfiction is a community, and when an author repeatedly reaches out to their community for connection only to receive silence in return it is very discouraging and can result in the author withdrawing from the community altogether.