Why is IGN SYSTEM A FAULT dispatch a NO GO on the A320 for ETOPS? In the A320 MEL if Ignition system A is under MEL the aircraft can't be dispatched for ETOPS. Why is it specifically restricted for ETOPS because even while conducting non ETOPS
 if we go into emergency electrical configuration we will still lose ignition system A.
Your assumption is incorrect. 

In ELEC EMER CONFIG (see your QRH - ELEC EMER CONFIG SYS REMAINING), Ignition A is the only ignition available and hence that would be your reason why IGN A is needed for EDTO / ETOPS.