Say that you got the opprotunity to work on this domain as a member of the pilot bathc with u'r current employer and you have been a potential resource and have explored a lot on this/gained good experience and exposure and have built u'r subject matter expertise and you wanna learn some thing new/confront new challanges and implement/utilize the knowledge u've built so far in some thing knew adding value to it where you can see your visability of your potential.This new job has come as a great opportunity in your way and you would not just contribute to this role/co. but also learn a lot from the excellent team you would be associated with.

Say a better opportunity where you have a platform to perform well/add value and learn more and grow with the co.

In the interview don't degrade your current employer for giving a reason for quitting your co.Remember to project that you have been with winners and this would elevate your image.Talk good about your co... this shows u'r professionalism/healthy relation ship with u'r co./potential and respected resource.