I've often thought about this, why were Zira and the outsiders banished? It was never talked about on why they can't turn their back on them. This leads me to believe that someone may have been murdered by Zira either when they first met during the end of Scar's reign. Or maybe Zira and the outsiders were accepted into Simba's Pride, and later on murdered Sarabi but before she could kill Kiara she was stopped and exiled by Simba.
It does seem a bit of a 'stupid' reason that they were exiled just for being loyal to Scar.