Interesting thought, DarylB. I know on a gasser the hp/tq both supposedly peak or converge at 5252 rpm, not sure on a diesel where the peak overlap is.
The formula for Horsepower (in our application) is:
(torque x RPM)/5252 = Horsepower
Horsepower is a measurement of work done in a given timeframe. Torque is simply an applied force, with no time element.
It comes down to where your torque peaks and how rapidly it falls off as you increase RPM. Diesels tend to have their torque peak at relatively low RPM (and don't usually turn high RPM) so will show low horsepower ratings.
Gassers normally have their torque peak at higher RPM. So they tend to have a higher peak horsepower rating, and horsepower and torque numbers are typically pretty close to one another because of it. (And right around that magic 5252 RPM.....)
Why the 5252? Well, that's a simplification of a lot of other things in the equations. More info can be found here:
http://auto.howstuffworks.com/question622.htm
That all said, remember the old saying: Horsepower sells cars. Torque wins races.