I think this is part of the problem when it comes to perception and reality. I find this especially true with those focused solely on weight loss through cycling versus just trying to get faster and stronger on the bike.

Again - just my experience and observation, but most cyclists in it just for weight loss, typically:

A) Typically ride at a lower effort. I think this is due to a couple of reasons. One, having been heavy in the first place, they have been trained to focus on the fat burning zone. While long-slow riding has its merits, it should not be your main riding style.

B) Because they are purely riding for weight loss, they do not push themselves like those who are trying to gain improvements in performance. Any hint of muscle burn, and they will drop into a lower gear. For those trying to improve performance wise, pushing through the muscle burn is part of a training regimen, which just happens to also trigger additional caloric burn. I blame the whole Carmichael training approach, where people began spinning on their bikes at higher RPM's. Applied correctly, and there is a place for this style of riding, but most amateur riders instead stay at the same lower cadence, but now have merely dropped the effort and energy needed to move the bike.

C) All too often, those who take up cycling for weight loss, do not realize that just because you are rolling around on a bike, that it is burning the number of calories they imagine they are burning. How often do you see these people stop mid-ride to take in a view, or coast down a hill.

D) Often those riding purely for weight loss, feed on the bike as if they are training for a race. Honestly, there is NO reason you need a Gu gel for a two hour ride on the bike path. Given the pace and time they are spending on the bike, between the Gu gel and Gatorade they are drinking, they can in some cases consume 1/2 of the calories they might be burning riding. Finally, (and I think this gets to the point about riding), they immediately begin to use riding as an excuse to stop watching their diets.

E) Finally - they do not put in the time or mileage necessary to lose the weight. They see a think cyclist, and do not realize that he/she just got done an 60 - 100 mile ride. They seem to think that most cyclists hit the bike path for an hour, and lost all of that weight.

I repeat, this is not universal, but they are my observations in relation to why so many cyclists end up losing the battle of weight even though they think they have suddenly become active.