Latin isn't spoken as a first language because an outside source (Celts) stepped in and altered it.
The language did not change of itself (heck, the later Roman period saw a revival of "true" latin, in an effort to keep it "correct" and "universal".)
An outside force changed it, and adapted it.
But the fact remains, we still use Latin to this day. (whether or not it is a "first language" the tangible evidence is still there)
So answer the question, especially given your recent answer: Why doesn't the ancestor still exist?
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.