Apart from Manchester City, other major teams have performed really badly in the last Premier League season. In the next season, those teams will want to build themselves up so that they can correct their mistakes and perform as well as Manchester City.
In every season, almost all the teams in the Premier League keep trying to improve themselves so that they can get very good achievements, as was done by Manchester City last season. And the thing for some of them who performed poorly last season was because every team didn't have nearly the same strength last season and every team also didn't have the same strategy in going through each season. So it's only natural that some of the teams in the Premier League performed poorly last season.