What could the reason be that my gum does not fully close after extraction as there is neither bone nor bit tooth left?

Probably not. Extraction sites vary in the amount of time it takes for them to close, ive seen them take a year, if u r having no pain i wouldnt worry about it, if it has been over 2weeks and the site is painfull i would go to the dentist that did the extraction and get an x ray.