Any soil attracting residue left in the carpet will accelerate resoiling. When people ask the question, they are comparing, to when the carpet was new, which of course they shouldnt be doing. If I am asked the question I also tend to explain along the lines of the worn fibres etc. However, I also add that the biggest cause of any rapid resoiling to a properly professionally cleaned carpet, is the fact that the home owner and family do not treat it like they would a new one. How many times have you cleaned a carpet and before you can stop them someone has walked over it without taking their shoes off. They wouldnt do that if it was brand new. Since I started to leave an advice sheet with clients, which includes, how to treat the carpet, especially during the drying process, I have noticed the carpets are staying clean far longer and when I go to reclean them they are not as bad as the first time.
Dave.