Writing mistake: The Ensemble Cast
As a writer, you learn how to make stories short, and avoid writing stories about large groups of people.
The Walking Dead could have been done in one season. Like any other television show, it is merely a collection of short stories strung together. If it centered on one character instead of an ensemble cast with episodes subdivided into adventures with each person, it would be shorter.
Also, they keep adding people to the storyline. If they'd stopped before adding people like Princess or Negan it would have been shorter. Even when we're not talking about fan favorites, I've seen random characters that pop up and immediately get killed, which maybe bloats the story unnecessarily. The more people you can eliminate from the plot, the easier it is to manage.
Also, the same things keep happening. They meet an enemy army, they fight, they meet a character associated with the enemy and they eventually join the team, and the older characters hate them. Then a new enemy pops up. If they stopped at one big bad army, the show would be shorter.