It is overwritten by default because it is an updated version, and there is no need to have duplicates.
Please note: We explicitly mention within our new installer that older versions will be overwritten (please see the attached screenshot).
A special note for earlier Nik Collection by DxO users:
If you do not wish to upgrade, and want to return to your previous Nik Collection by DxO version, please first uninstall the new Nik Collection by DxO installation using the uninstaller on your computer, login to your account here and download your version of Nik Collection by DxO, then install the software on your computer once again.
On Mac OS: