Start by explaining what you liked about your current employer. Focus on the positives! Next, share that you want to leave to pursue better opportunities. You really don't need to get into the details of what's missing from your last job. Focus on what you want to accomplish in your new role and how this position will help you get there.
"I am interested in working for you because there is so much room for growth. There is so much opportunity at your company, and I think I will learn so much."