When you transition from running to cycling it can take a while (years?) to build up your legs to the point where they can stress your cardiovascular system as much as running. It's also easier to take a break while cycling and ride with a low HR. Taking a break in running is called walking and most just want to finish the run so the minimum level of effort to run is higher than with cycling.

I think it also depends on your body type. A skinny efficient runner/cyclist will react differently than a more powerfully built athlete. To get a high intensity running workout without pounding your body mercilessly it helps to be light. On the bike it doesn't matter so much if you're heavier, you'll just go slower.