I hear many people asked, why a webcam can out perform cooled CCD, DSLR, DC, etc? Their assumption is, other cameras are more expensive, higher quality, with lower noise level than that of a webcam, so they probably should perform better.
So, what's good for a webcam? the single most important advantage is high frame rate, other than that, smaller pixel size could be another one. Okay, let's go into a little bit more detail.
For high resolution imaging, we're dealing with very large focal ratio in terms of f/20 to even f/50, and therefore, we will need relatively long exposure which at the same time means noise. You will say a DC is cleaner than a webcam here, yes, you're right, but it's only right for a single frame only. The most important thing which makes webcam shines is image stacking. By image stacking, we average out the noise, and the noise amount is inversely propotional to the number of frames that we stack. When using a DC, you can do 100-200 frames at most, but with a webcam, you could get 1000 frames rather easily.
Second is the pixel size, for most webcam, you can do f/20 and you can the proper sampling, but with film, we will need f/60 or even more.
High frame rate also helps you to freeze seeing condition, when we got a lot of samples (frames), we could selectively throw away ugly frames and stack only the best of the batch.
Of course, the above is a simplification of the situation but this is about what's the most criticial reasons.
For the above, you can easily see that why those expensive Lumenera and DMK are gaining popularity among serious imagers. They provide even higher frame rate and they can deliver raw frames uncompressed.