因为在这个句子里,强调的是 我迟到了 .而过去完成时是以过去强调现在,句子中强调的是过去.
看看这个你贵明白些：
过去完成时的谓语动词构成
I/ we/ you/ they had worked
he/ she/ it had worked
1.表示过去某一时刻之前已经完成的动作,常与由by,before引导的时间状语连用.
We had learned 5000 words by the end of last month.
到上个月底为止我已经学了五千个单词.
I had finished the composition before supper.
晚饭前我就已经把作文写完了.
2.表示过去某一动作之前已经完成的动作,常与由when,before等连词引导的时间状语从句连用.
When I woke up it had already stopped raining.
我醒来的时候雨就已经停了.
I hadn't learned any English before I came here.
我来这儿之前没学过英语.
常用于引导这类状语从句的连词有：when当...的时候,as soon as一...就,before在...之前,after在...之后,until直到,等.
3.用于宾语从句或间接引语中
I wondered who had taken the umbrella without permission.
我想知道谁不经允许就把雨伞拿去.
He told me that he had passed the exam.
他告诉我他已通过考试.
4.某些动词的过去完成时表示一个打算要做却没有做成的事.
I had intended to come over to see you last night,but someone called and I couldn't get away.
昨晚我本打算来看你,但有人来找,脱不开身.
We had hoped to catch the early bus,but found it was gone.
我们本希望搭早班车,却发现车已开了.
这类动词有：intend打算,hope希望,plan计划,mean预定,want想要,think想要,等.
5.用于表示与过去事实相反的虚拟条件从句中
If you had come yesterday,you would have met him.
如果你昨天来的话,你就已经见到他的面了.
She would have come if she hadn’t been so busy.
要不是这么忙的话,她就已经来了.
