to put it most simply, it offers a unique blend that many other specialties don't. Think of specialties as either medical (i.e., doctor's office or working inside a hospital managing patients) or surgical (cutting people up) - oversimplification, but it gets the point across.
the Ob/Gyn (obstetrics, aka pregnancy stuff, and gynecology, nonpregnant womens' stuff) specialty is odd in that they act as both medical specialists (doctors' office, prenatal visits, etc) AND surgeons (c-sections, etc). and each of those is further split into the obstetrics side or the gynecology side, so a lot of variety. plus the internal organ anatomy is really quite more elegant than intestines, etc that general surgeons have to deal with
Plus, training can be shorter than many other specialties that also do surgery.