What are the reasons for the decline in quality of anime sequels? Why are second seasons often disappointing?

Many anime have a great first season, but the quality drops significantly in the second. There are too many examples to list... but I want to ask, what is the actual reason for this?<br><br>My guesses:<br>1. The first season already used up the best parts of the original source material.<br>2. The production committee rushed the schedule.<br>3. Changes in key creative staff.<br>4. Budget issues.<br><br>What do you think is the main reason? Are there any counterexamples—anime where the second season was better than the first? I can think of <i>Mob Psycho 100</i> and <i>Attack on Titan</i>.