Why is the hash part of the URL not available on the server side?

The hash component is not passed on to the server but it is extensively used on the client side. Specifically, in single page applications, the text following a hash is used to represent state of the application as different routes. Thus, what happens is: following an initial request to the server which serves the 'home' page along with additional js files which include client-side routing logic such as the router, whenever the user navigates anywhere on the page by clicking an anchor tag, only the part of the URL following the hash component is changed. This prevents a GET request to the server and in response to this 'onhashchange' event, the content of the single page application can be updated depending on the exact route.