Basically, it’s “too good” to be mass produced. This has drawn the attention of skeptics who claim there were likely other technical issues that lead to this decision. The Russian military currently has 12 of the aircraft ordered.The Su-57 is considered to be one of the best aircrafts produced in the world. Consequently, it does not make sense to speed up work on mass-producing the fifth-generation aircraft.
http://www.businessinsider.com/russia-a ... ion-2018-7