Basically, the star hasn’t wanted to hit upon a brilliantly crafted Negan storyline on the page, only to be majorly disappointed if the showrunners and writers would take things in a different direction.
What Morgan doesn’t know can’t hurt him or Negan, though The Whisperers can.
That’s fully understandable, but Morgan’s other reason for not reading is even more justifiable, as the TV series continues to divert its narratives away from how Robert Kirkman planned them all out in the source material.