why is my i cloud not backing up my photos?

You have the ability to downgrade your iCloud if you wish. You are the only one that can increase that plan. See this for downgrading. Downgrade or cancel your iCloud+ plan - Apple Support

If you go to Settings><your Apple ID>iCloud, if you have Photos turned on, then you are using iCloud photos and they are not part of your backup. If Photos is not turned on, then they are part of your backup. Understand that the free iCloud storage, 5GB is not much space, especially if you are using iCloud for backup. Make sure you have ample space in iCloud so you do not lose anything.

Content that is saved in iCloud is not backed up. So Photos, Contacts, Calendar that you are using iCloud to sync is not saved in your iCloud backup. That is by design.