Why is a primary far less common and takes much longer to complete than secondary succession ?

Secondary succession generally takes place faster in comparison to primary succession as the substrate is already present. In the case of primary succession, there is no soil and it consumes much time to form.  

Primary succession takes place in mainly lifeless regions, that is, the regions in which the soil does not possess the tendency of sustaining life as an outcome of various factors.  

On the other hand, secondary succession takes place in the regions where a community that previously prevailed have been eradicated and is epitomized by smaller scale turbulence, which do not eradicate all the forms of life and nutrients from the environment.