When we talk about Homo ergaster and Homo erectus it doesn't do justice to the significant variation among, and geographic distribution of, the fossils that are grouped under these species names. It's hard to know which variation can be accurately attributed to intra-species variation or sub-species or even separate species.
However, what is clear is that there is a great deal of variation in Africa, Asia, and Europe around the time of the "eretines". My feeling (back when I was a student) was that the archaeological evidence suggested an ongoing adaptive radiation of sorts, rather than the classic picture of a steady and relatively homoegeneous trajectory towards modern humans. But a lot of new research has been done since then that I'm not up to speed on.
Prior to the displacement and extinction of the remaining human forms there was already a reduction in the number and variation of Homo species. Even within our own species, there was much greater variation some 200-150kya.
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.
You just have to look at the incredible lack of hominid (great ape) fossils for the last many million years of years to see know how difficult it can be to piece together an evolutionary history of a taxonomic group.