Additional description for John Smith's answer from the official documentation. To understand why it is.
Main host goes away
If you are adding virtual hosts to an existing web server, you must also create a block for the existing host. The ServerName and DocumentRoot included in this virtual host should be the same as the global ServerName and DocumentRoot. List this virtual host first in the configuration file so that it will act as the default host.
For example, to work properly with XAMPP, to prevent VirtualHost overriding the main host, add the follow lines into file httpd-vhosts.conf:
# Main host
<VirtualHost *:80>
ServerName localhost
DocumentRoot "/xampp/htdocs"
</VirtualHost>
# Additional host
<VirtualHost *:80>
# Over directives there
</VirtualHost>