Because your system is still on hyper-aware status. Your body is still geared up in that fight or flight pattern and it takes a long time for it to let down. You body has to re-learn how to do that, and only after your mind starts to relax, and then nothing happens for a while, will your body start to slowly and cautiously unwind as well.
It really doesn''t have anything to do with you, IFKWIM. It''s a eons-old response built into your body to keep you alive.
Which is a very long way of saying that darned 4-letter word: time.