Assuming that the export news of Su-57E is actually factual - I suppose that the customers are in fact countries which would be deemed rather "politically sensitive" for China's J-35E (provisional designation) - And that Russia's current political situation (i.e. getting sanctioned left-and-right by the West) inadvertently granted them freer hands in such arms deals.
Otherwise, it'd be just inertia (namely, having currently operating largely Soviet/Russian fighters, and that continued support from Russia for their latest fighter jets would be easier).
Iran, Vietnam and Algeria comes to mind as probable customers.