As a person who has had first hand contact with many such cases at the darul iftaa, I would say the major reason people marry secretly are : "The first wife is not groomed enough islamically to accept a polygynous marriage. While many women will boldly say I rather have talaq from him than allow him to marry a 2nd. Other would say I rather have him do zinaa then have a saukan in my house!!! (subhanallah I do face palm every time I hear such responses..knowing we are an ummah whose aslaf most if not ALL had polygynous relations) Many are not willing to accept that he doesnt need their permission. The minority cases are such that the man is simply a leech who wants to exploit the scenario.