There is nothing stopping you from riding your bike at an intensity where you burn 1000 calories an hour. I think all that you've discovered is that doing high intensity work has a much greater benifit per hour.
In terms of being more wiped from running vs. cycling I would think this is due to training specificity. You're just a better runner, you can maintain a higher intensity with a lower heart rate while running, thus you're able to more effectively exhaust yourself.
Think of it this way: I'm sure that you could find a hill to ride your bike up that would take you about the same amount of time as that original 10 mile run. You could also find a speed to go up it that would leave you feeling just as exhausted and spent as the run did. The main problem of course might be that you just aren't in good enough biking shape to go that fast, but that's not because cycling is inherently easier than running.
Another thought experiment: If I say that both rowing and x-c skiing are "more difficult" than running how do you counter that statement? I have plenty of personal experiences to back up my claim.
I personally think that people tend to think of the sport(s) that they have learned to trully push themselves in as the one(s) that are the "most difficult." I know that's what I did.