Wet Carpet Cleaning = Detergent Residue                                                                    Zero Residue Advisory
Wet carpet cleaning methods, including some that claim that they are "residue free" and some that have the word "dri" in their name can leave soap residue in carpet. Wet methods can and often do attract new soil.

After carpet is cleaned with a wet carpet cleaning method the water evaporates from the cleaning solution and the detergent dries and re-crystallizes within and throughout carpet fibers. During the drying time, (2 hours to 6 days), the water (H20) evaporates from the "cleaning solution" and the detergent remains. Detergent residue can attract new soil, trigger rapid re-soiling and cause carpets to "ugly-out" and become matted down. Simply put, detergent residue (soap) attracts new soil. Blogs

Returning Spots - Soap and Water
Detergent residue from any wet carpet cleaner. Most wet carpet cleaners cast the blame for returning spots on "wicking", sometimes referred to as wick-back. Nevertheless, the phenomenon of "wicking" is usually nothing more than a scripted excuse used by professional carpet cleaners to get out of the responsibility of leaving soap to dry in carpet. Again, spots do not mysteriously reappear. Meet chemist

160-200 Fahrenheit Is HOT - Removal of Stain & Crush Resistant Materials   
Hot water can dilute and/or remove valuable stain resistant materials in addition to crush resistant properties from carpet. Beware of wet methods that claim they remove dirt with extreme temperature without detergents.

Health, Allergies, Mold And Carpet Cleaning
Steam cleaning can force water to gain access to carpet padding. This is not only disgusting, but wet carpet can (in some rare cases) be hazardous to ones health. High levels of humidity stimulates growth of mold.