First of all, Sumitomo's management philosophy of "public interest and public benefit" and "never pursue easy gains" was put into practice by Sadakane Iba, who served as the general manager of the company.
Usually, in such pollution disputes, a measure called "sympathy money" is used, but Iba knew that this only postpones the problem.
So he planted 10,000 trees each year in the mountains that had been dried up by their mining operations and purchased some uninhabited islands to move the refinery to places where they would not be harmed.
It was these attitude of Sumitomo that Shozo Tanaka praised.
Another reason was that Besshi was in Shikoku. Not to sound too personal, but it was a story from another prefecture. On the other hand,Ashio Copper Mine was located in Shozo Tanaka's district. Therefore,he needed to protect the livelihood of the local people at all costs.
This is the reason why Shozo thoroughly pursued the issue of mineral poisoning in the Imperial Diet.