由于这个原因 for the reason
The reason( why.) is that ...（ ...的）原因是.
括号里为定语从句,修饰先行词the reason ,that 从句是接在is 后的表语从句,可以说it is because ,“这是因为” 但不可以说the reason is because “原因是因为”讲不通
This is the reason why / for which he was late.是按正规语法走的.
this is the (very) reason that he is is late 和 this is the reason he is late 在口语中也可以接受,但在考试中一般比较规范.