Su-57 is still raw. It probably has higher maintenance costs due to low-observability surface coatings, maybe even requires hangars. Its intended engine is still in development and testing.
1. Engine still not finished
2. More expensive to make
3. More expensive to maintain and operate
Buying 12 of them with transit engines right now makes sense and allows to develop tactics specific to the plane, as well as keep testing and perfecting other systems prior to mass production. Also, mass-production is not yet critical since, according to the official's statement, Su-30s and Su-35s are more than adequate for the tasks at hand. In addition, other important projects require funds right now. So no rush, not ready, not critical, not the most optimal use of resources.