I'm not going to explain in great detail but there's a lot that goes into why you cannot produce power at a high heart rate. Most of it has to do with muscle type.
I for instance suck at distance, i can move an incredible amount of weight for low reps. However my muscles fatigue quickly with high reps. It's the same with cardio. Quick sprints vs. long distance, my muscles fatigue fast but can produce a lot of power in short burst/distances.Fortunately you can train your body for certain things, you will not be as good as someone who is genetically built with slow twitch fibers though. Also, get your V02 max taken at osu. That is another factor that may be hindering your performance.
Are you taking any stimulants?