The extension is not added, when it has no require-dependency for content blocks in its composer.json.
This should not happen, as we can not be sure if everybody has this dependency set. For example, if you have a composer.json for the whole project, and the dependency is set there, but not in the sitepackage.
I added my custom sitepackage and this was not shown inside the dropdown for selecting the host extension. Reason could be https://github.com/krausandre/content-blocks-gui/blob/c2900d339519473e7dd7f76555dca22a6e2e81a7/packages/content_blocks_gui/Classes/Utility/ExtensionUtility.php#L59
The extension is not added, when it has no require-dependency for content blocks in its composer.json.
This should not happen, as we can not be sure if everybody has this dependency set. For example, if you have a composer.json for the whole project, and the dependency is set there, but not in the sitepackage.