Explanation/Reference:
The Set-SPSiteUrl cmdlet adds or changes an URL mapping for the site. The Set-SPSiteUrl cmdletonly applies to the root site collection for a host name that is, http://www.contoso.com. This cmdlet cannot be directly run against a managed path site collection underneath the root that is, http://www.contoso.com/ sites/test.
When you create a new hostname site collection, the default alternate access mappings will still exist but cannot be used. Use Windows PowerShell commands to manage URL mappings for host-named site collections.
Add a mapping to an existing site:
Set-SPSiteUrl (Get-SPSite 'http://teams.contoso.com') -Url 'http://teamsites.contoso.com' -Zone Intranet Reference: Host-named site collections versus path-based site collections