Plone requires that a content-type must be non-folderish in order to be used as default page for a folder.
In Plone 5: content-types created TTW are by default folderish (and there is no way to change this behavior) and can not be added for that reason as a default page for a folder (which I consider a bug).
The same is true for folderish content-types created through Python code...in my case I have a folderish page type and I want it to be used as a folder's default page -> not possible.
The default behavior is to not let folderish types be set as default pages IF the type allows other types inside, but that can be overridden. See the logic here: https://github.com/plone/plone.app.content/blob/1302345ef82a6246ef6ce30e08e01ef9b4ea5033/plone/app/content/browser/selection.py#L97. You can make a folderish type be included in the default page options by adding it to the
plone.default_page_types registry setting.
You can change it in ZMI /portal_types
Taken from memory: I think you change it to plone.dexterity.content.Item
(from 'Container' )