The reason why is because you want to vary what is shown in the second field based in what is selected in the first field. This can’t be done using ACF without adding your own custom JavaScript to the admin and to be honest, I can’t help you do this. You’d need to filter what the ACF JS is sending in the AJAX call.
The best work around for this would be to create conditional fields based on the first field that show posts from the selected type, and I’m going to be honest, unless Type is a taxonomy even this may not be possible to filter following the link that you posted. There have been many posts here on the forums about people having problems filtering relationship field choices by another custom field.
