Why are recumbents so slow uphill?
Bike geometry and bio-mechanical topics have been discussed by others already, but I wonder if a large part of what you're observing is a relative difference between speeds on the two types of terrain?

Think of it this way:

You ride a "regular" bike on the flat. Your ultimate top speed is essentially governed by aerodynamics. You reach a speed where the force of the air you're pushing against equals the force you put in.

When you ride a recumbent under the same conditions, it's more aerodynamically efficient, so your top speed is higher. 

Makes sense.

Going uphill it's a little different.

A larger part of your input energy goes in to overcoming gravity. Which is why your top speed will be lower.

Riding a recumbent uphill, you're still overcoming exactly the same amount of gravity, so aside from other mechanical advantages a recumbent may or may not give you in this situation, your top speed will be roughly similar to riding any other type of bike. The aerodynamics play a much smaller part in the equation..

Summary : the relative difference in the two top speeds is going to be greater  (and more noticeable) on a more aerodynamically efficient machine.

Caveat: I'm neither a scienctist, nor a recumbent rider :P