In his posthumously published memoirs, Valeri Legasov, the First Deputy Director of the Kurchatov Institute of Atomic Energy, revealed that the Institute's scientists had long known that the RBMK reactor had significant design flaws.[16][17] Legasov's death from suicide, apparently as a result of becoming bitterly disillusioned with the failure of the authorities to confront the flaws, caused shockwaves throughout the Soviet nuclear industry and the problems with the RBMK design were rapidly accepted.[18][19]

- Wikipedia

The design of the RBMK and the lack of criticism were due to political reasons. Similar to how today we can't update or replace reactors.