【送分题】no one here believes the reason (that) he gave for his

【送分题】no one here believes the reason (that) he gave for his lateness
为啥选that呢?不是说先行词是reason时 定从的关副是why或for which吗?
我真得崩溃了 明天就考试了 学位英语
我立马儿采纳 我这些天问的所谓【送分题】也没送分 而是百度送你们的分 因为我财富值全用光了 所以不能悬赏了 请海涵