What is it that you are missing from your current opportunity? If given this new position, would you be receiving all that you need from this new opportunity?
Be sure to remain positive when you answer this question and avoid speaking poorly about your current supervisor or company.
"For me to stay in my current job there would need to be less 'on-call' work and a higher pay rate. Currently, I am on call 3 weekends per month and my current company does not offer a competitive pay rate for my level of expertise."