Why are diamond push-up so hard?

Diamond Pushups are hard because they rely on triceps and limit the involvement of the shoulders and chest. The triceps are also in an awkward position and unable to exert its full strength.

Are diamond push ups better than normal push-ups?

The diamond push-up is one of the most effective triceps exercises. The unique hand position of this bodyweight exercise activates your triceps brachii more than a standard push-up.