Please help with this email

I emailed my manager to ask for vacation. This is what I got this morning:

I will have to investigate. We have already received a number of requests for July 3rd. The will be granted in the order they where received so I will let you n\know tomorrow if there is a problem or not.

Since I only work in this company for one month, and it is my first time to ask for vacation, it is a surprise for me while I am reading this email.

Do you help me to analysis why my manager wrote a email like this to me?

Thank you so much!