If you portray that you are very happy with your current job, then they might ask you this question. I was in a very similar situation and I will include my answer as the example. You can use other reasons, but make sure it doesn't contradict you being happy.
This is simple and direct. It answers the question without including unnecessary excuses. It also shows my desire to grow, learn more, and improve myself. This is a good indication of a good employee so using an answer like mine is a safe answer.
Do you need to Learn English Faster? Do you want to speak English fluently?