There aren't any easy (accurate) answers to these sorts of questions, which is also why they are such fascinating areas of research. Understanding all the factors and events involved is very difficult with the type of information available.
Speciation is only something that can be determined after the fact, anyhow. Species are entities we use to discuss the combination of innumerable bits of historical happenstance. Our being the only human species left might be due to little more than luck.
Contingency could well be involved. But it's not particularly easy to decide what constitutes a separate species, even with extant organisms. The categorisation of fossils into separate species is probably not as interesting or important as trying to understand what can be deduced from the amount of variation between and within populations.
Speciation is only something that can be determined after the fact, anyhow. Species are entities we use to discuss the combination of innumerable bits of historical happenstance. Our being the only human species left might be due to little more than luck.