How to answer the "why are you looking to leave your current job" question after receiving a promotion and only having been with the company for a year?

I work for a doctor's office as an office supervisor. I was promoted to the position about 4 months ago after only having been with the company for ~8 months as a receptionist.

I'm very unhappy with upper managements lack of communication as well as lack of support they provide to supervisors, which in turn makes it very hard to do my job and manage my staff. None of the staff feel supported by any of the doctors or upper management. The company has lost several employees with no thoughts of hiring new ones, so we are always running short staffed. I've tried to suggest changes and ideas to management higher than me and it is always met with resistance and basically my ideas are useless. The company as a whole only seems interested in making money and keeping it to themselves, i.e. the doctors and practice manager.

They just gave me a pretty hefty raise after my 90 days (I believe this is only because another supervisor who got promoted just before me put their 2 weeks in and they're trying to make sure they keep me) however I still feel that for my mental health I need to leave. I have an interview tomorrow at another doctors office for just a front desk receptionist, but through inside knowledge I know this position pays ~$1 less than what I'm making as a supervisor at my current company, with less being taken out for health insurance so it potentially equals out and will be a less stressful position.

I obviously do not want to slip and say anything negative about my current employer, but unsure how to answer that question positively?

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