Because it died out. Maybe there was an environmental change that the species could not adapt to. Maybe other species (such as some of the species descended from the original species) ended up wiping out the ancestor species.
Species go extinct all the time. It's not particularly surprising that one of our ancestor species died out, since the majority of our primate ancestors have died out.
Highball apparently didn't know this.
That wasn't to you. I have a firm grasp of the reasons supposed. I was asking highball to clarify his dim point.