how do I tell interviewers why I’m leaving my job without badmouthing my employer? — Ask a Manager

FAQ: Is it badmouthing an employer if it is the truth? – As a general rule, it’s best to avoid answers about disagreements or drama when talking about why you’re leaving your current job, even when you’re not the source of them. – When interviewers ask why you’re leaving your current job, it’s better to avoid answers about disagreements or drama, even when you’re not the source of them. – Cite the fact that you’ve been at your current job for at least a few years, and you’re ready to take on something new. – Look for the most neutral, low-drama way to explain the situation. – Avoid subjective assertions like “my boss was a nightmare” or “the leadership was in chaos”. – Talk about what’s drawing you toward the new job rather than away from the old one. – Whenever you can, go for bland and unmemorable answers.
Summary from www.askamanager.org