There are many small reasons. In no particular order:
- Backup upgraded with Ware's return
- QB downgraded from Smith (2017 Smith, at least) to Mahomes
- Offense went from 3 quality targets (Hill, Kelce, Hunt) to 4 with addition of Watkins
- New OC
- Injection of RBs above him in the rankings - return of David Johnson, rookie Barkley, Kamara (for many people), Fournette (for many people), and Cook (for many people)
- Off field concerns (2 alleged incidents this offseason)
Then there is the Jekyll-Hyde issue:
- He started strong in the first 5 games - 97/609/4 rushing (6.3 ypc) and 16/166/2 receiving (10.4 ypr), with 4 100+ yard rushing games.
- But then he disappeared for 7 games - 96/321/0 rushing (3.3 ypc) and 22/201/0 receiving (9.1 ypr), with 0 100+ yard rushing games.
- Then he played well for the next 3 regular season games - 78/362/3 rushing (4.6 ypc) and 14/88/1 receiving (6.3 ypr), with 2 100+ yard rushing games.
- He had 1 carry in game 16 but sat out the rest.
- Then he had a forgettable game in a playoff loss - 11/42/1 rushing and 3/5/0 receiving
That middle stretch is troubling. What happened? More importantly, could it happen again? How likely is it that Fournette, McCaffrey, Cook, Gordon, or Kamara would have a stretch like that?
Perhaps none of the stuff above is significant on its own. But add it all up, and it impacts perceptions.