This seems counter intuitive, 'Exar Kun was winning the war until he started losing'. It seems Kuns defeat was due to successive defeats at Onderon and Ossus at the hands of the Republic. We also have to remember MtI is not in this fight and neither is Qel-Droma. If Kun is so lost without them then surely all MtU needs to do is bombard Yavin 4?
He's not lost without them. The Mandalorians made a foolish attack on Onderon after Ulic was captured. They had absolutely no support from Kun's forces, and they were defeated. Then Ulic lead the Republic to Yavin 4, where the Jedi imprisoned Kun's spirit. Ulic's capture caused the end of the war. Without Ulic, Kun has no need to worry about anyone leading the Mando's to him. He can do as he pleases. We saw Exar Kun have his way with the galaxy until Ulic was captured and the Mando's were defeated.