Because you have to compensate for the negative
effect of the dead spot area. When riding on level
roads this has no effect except for the loss of the
power that you could be applying if you could make
use of that non power application area. Is there a
better explanation.

Gravity?

Maybe pride and mechanical consideration too. I wouldn't want to deal with the gearing required on my bike to maintain 100 cadence up some hills (39-27 means you have some pretty rough shifts in there, as few courses are 100% climbing) and I wouldn't be caught dead riding a triple up anything other than a Vuelta-style mountain.

Fitness. You don't have the fitness to maintain that cadence and still have the power to keep moving in the gears you have. Same reason that fitter cyclists push 25 mph at the same cadence you push 17 mph.

Problem with this is that even the most fit riders (i.e. someone like Lance) who are known for a high cadence climbing style still seem to climb at a lower natural cadence than they time trial at, even though both events are supposedly equivalent, high powered efforts.

I think Perfection may be on to something here. On a flat ride, your momentum isn't slowed very much by the dead-spots in your pedal stroke...on a climb, inefficiencies show up more because you have less momentum. If you have a flaw in your pedal stroke that causes you to lose momentum, you want to decrease the number of flaws per minute, so you pedal more slowly. You don't want to pedal so slowly that you increase muscular tension to the point that you get diminishing returns. But, perhaps slower allows you to make some pedal stroke adjustments to smooth out the power application around the continuum of the crankarm motion, i.e., you pull back more effectively, scrape at the bottom better, shove over the top, etc.? Maybe?

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Quid quid latine dictum sit altum videtur
(That which is said in Latin sounds profound)

everyone's bike is too light. Didn't we just go over this :-)

Frank

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Frank,
An original Ironman and the Inventor of PowerCranks

I agree that my bike is too light. The problem however is that my ass is too heavy. Power/weight ratio is inversely proportional fatcarcass/bikeweight ratio or something like that.

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That is just one more group of people that should be thrown screaming from a helicopter- George Carlin

Having too light a bike explains why that guy on a 50 year old Schwinn (the kind with a fuselage-gas-tank for a top tube) passed me like I was standing still while we were going up a mountain. He must have been doing 40 mph! But, I noticed he had those heavy PowerCranks and I had on my stupid lightweight Dura Ace cranks!

Seriously, though, there's no question that I climb better on PowerCranks, so my PowerCrank stroke is more efficient. The difference between the cranksets, in terms of rpm, is nil while climbing, and sitting up is best for me when PowerCranking. I have noticed, as I train longer on PC's, that the differences in speed are less (although the speed is greater on either set). I'm finally carrying over some pedal stroke technique benefits to my regular crankset pedal stroke. To quote a really good climber (compared to me, not compared to a tour rider) I rode with in the mountains a couple of weekends ago: "You hurt me real bad". This guy used to fly away from me on the climbs, which I always attributed to his 25lbs. lighter body weight. Now, I drop him fairly easily. I notice we both pedaled right about 80rpms, I just push a higher gear. On the flats, we both normally pedal about 95, but, when I put it in a bigger gear and drop to the low 80's, my HR drops a little and my speed increases, and he begins to have trouble keeping up.

One other thing about climbing, if you sit up and back somewhat compared to on the flats...you may be engaging your gluteal muscles more on the climbs using this positioning. This increased muscle mass may be harder to use efficiently at higher rpms. So, maybe we slow our rpms some to allow for more efficiency for this reason.

Anyway, this reinforced what I was told by a coach last summer (before I trained on PCs), that lower rpms would be faster for me during time trials. I used to pedal circa 100 during TTs. I had my best time last year at a TT where I pedaled just above 80 the whole way. I had my best triathlon split in a 40 K doing the same low-80's rpms. This used to hurt my knees, but, not any more.

I think trying out different rpm ranges may show some people what they thought was best for them may not be. Some people may need to increase their rpms, some may need to decrease their rpms. If you notice you climb better at low rpms, I'd suggest trying lower rpms on the flats, too. This lower climbing rpm may be a sign that shows your pedal stroke is more efficient when slower, and as long as it isn't too slow, it may be worthwhile to experiment with this somewhat lower rpms on the flats. I think it is a matter of finding out what works for your particular system, and realizing that it may change over time.

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Quid quid latine dictum sit altum videtur
(That which is said in Latin sounds profound)

Thanks to everybody for thinking on the subject.
I think the answer lies in the momentum/dead spots area. The only difference between a 1 hour time trial and a 1 hour climb is (physically) the gear you are riding in. A lower gear (climbing) means that the momentum fluctuations from your pedal stroke are amplefied to the rear tire, so the fluctuations in proppelling force are greater when climbing. Especially if you have a rough pedal stroke, this can result in a 'jerky'and thus inefficient ride. Pedalling more slowly makes it easier to pedal smoothly, so maybe that's why most (top)riders use a lower cadence when climbing.
I agree that a lot of recreational riders climb at low cadence simply because they haven't got a low enough gear on their bike. Especially those who are ashamed to ride a triple. But obviously these 'tough' guys have missed the point completely and can not be taken seriously.