Fitbit just hasn't included it in it's ios app. I believe the feature exists on android. There have been numerous feature requests for it which i believe have all been denied.
It does appear that apple has included the option since ios 9 https://www.redmondpie.com/third-party-apps-for-ios-9-with-quick-reply-support-list/
The android app seems a little more advanced than the ios version and i'm not sure if it has anything to do with google taking them over. There is no widget for the ios app nor is there quick replies.
Maybe @KateFitbit can better explain why the functionality hasn't been included yet
