Why is CubeBackup not listed on the G Suite Marketplace and can it be installed directly through the G Suite Market?
G Suite Marketplace
The G Suite Marketplace is a great place to find useful tools to facilitate your G Suite management and extend G Suite functionalities. App installation is easy and straightforward. However, only web apps can be added to G Suite Marketplace. Desktop apps, or any apps requiring local storage access, cannot be listed on the G Suite marketplace or be installed via a web browser. This is due to the technical limitations of web browsers.
Google Service account
So, unlike other web-based G Suite Apps which can be installed automatically with only a few clicks, CubeBackup requires users to manually create a Google Service account and authorize that account for Google APIs to access the whole domain. This initial configuration may be a little more complicated, but it also gives greater control over the Service account by allowing you to see API overviews and set quotas for different APIs.
More detailed instructions about the initial configuration are available at CubeBackup Initial Configuration.