You must request open access in order to access shared NSUserDefaults. It’s stated directly in the App Extension Programming guide:
By default, a keyboard has no network access and cannot share a container with its containing app. To enable these things, set the value of the RequestsOpenAccess Boolean key in the Info.plist file to YES.
Be sure you change the RequestsOpenAccess field to YES. You’ll find it in keyboard’s Info.plist > NSExtension > NSExtensionAttributes > RequestOpenAccess. Then remove the keyboard in Settings, delete the app, run it again, and add the keyboard again. After you add it, tap on the keyboard name and then flip the switch to enable Allow Full Access. You’ll need to instruct the users to follow those same steps to grant access (and reassure them you’re not evil), otherwise it simply will not work and you’ll never get the data that’s stored in your shared container. Note that in iOS 8.3+, if the user hasn’t enabled full access the keyboard will be able to access the shared container, but writing to it will not save the data, for security and privacy purposes. In 8.2- you can’t access that data without open access granted.