He’s easily the best villain in The Walking Dead, so Negan’s presence needs to be as powerful as possible, for as long as possible.
(It wouldn’t be good for anyone if the TV show caught up to the comic storyline by Season 8.)
There are good reasons why a decision like that would be made.
One, after years of slowly moving through the first chunk of the comics, The Walking Dead hastily plowed through Negan’s build-up on the way to the cliffhanger in last season’s finale, so expanding and stretching the “March to War” effort could be a way to re-balance the pacing.