Because it is made in Russia, and no its not about the quality it is because of "just in case measures", in case the west decided to slap some embargo to Indonesia again. The only reason Indonesian air force have SU-27 and SU-30 is because of embargo.But why would Indonesia want Su-35 of all other fighter jets.. KFX is gonna be introduced into service in around 2026 why burden yourself with that poor performance fighter jet like Su-35.. If Indonesia wanted to bolster it's fighter jet inventory than go for Rafale which IAF has already signed letter of intent for but in the main time skip Su-35 but if it is for good gesture than I guess you can drop could of billions on it to charm the russians but as deterence is doesn't really offer much.. The current Indonesia inventory is much better and su-35 will not provide an upgrade