It's not only because they broke out around the same time. They both made grunge popular - grunge being an image and not necessarily defined by a music style. They are Seattle based, wrote angst-filled lyrics, rejected the glam band era of the 80s in appearance, lyrics and sound. Thus, by definition they are a grunge band. Does not mean they have musical similarities (beyond distorted guitar sounds and self-doubting lyrics) with the Nirvanas, Screaming Trees, Malfunkshun, or Green River to name a few.