Don't know why some posters are being so unpleasant.
Change makes many of us feel vulnerable and unsettled - even changes we actively seek to bring about.
Of course you're feeling a bit wobbly. It's human.
And it's easy to look at a situation where another staff member has been asked to stay and you haven't and think, 'Why not me?'
But ... it's a dead-end as a thought-process.
There could be many, many reasons. Lots listed by other posters.
For you, the crucial thing is to focus on your future.
You've learnt a lot in this school - skills, ways of doing things, etc - and, crucially, you have learnt that you have preferences, strengths and weaknesses.
You learnt that this current school possibly wasn't the best fit for you.
Time to move on, keep looking, keep building your skills and your repertoire.
Which you have a opportunity to do. And that's a good thing. It's an exciting thing. And it's an opportunity.
You need to think of it that way because it's the truth and because it will help you ace the interview.
If you dwell on the, 'why not me?' it will come across at your interview - and you don't want that.
You need to be focused - like a ninja - on why you are really excited about the thought of joining the team at this new school. The new opportunities to develop it offers. The things that make it unique and why you find that very attractive.
By all means, you can ask your old school (& it is your old school now,) what areas they think you could seek to develop and put work into. But perhaps do that after you have a new place. And do it in a positive way, as a pointer to skills-development.
And bear in mind that schools are ridiculously stretched at the moment. I've never experienced anything like this. The situation is extreme. So, you'll be asking for something from stretched people - and anything they say should be taken positively and not as negative feedback.