There are a number of reasons, here is one.
You can not reuse the name of a previously deleted site ... Something you need to think about if you ever decided to delete a site.
However maybe it would be great to buy a www.domainname.com
You can then your google site with it.
Then it really doesn't matter what your google site url is.
Alternatively you could create a similar name.
Eg if your site name was ...
Create this name instead ...
Let me know how you get on?