>Why were the previous expansions out of Africa dead ends?

Richard Dawkins would say that descendants are common. Ancestors are rare. Most populations of all species leave little or no genetic trace.

The first human radiation was georgicus... 1.8mya. That is arguably the original homo species. Arguably pre-homo, if not for some long legged or large brained individuals in the tribe.

They may be ancestral to later Eurasian species of homo... even the erectus lineage as a whole. But likely not.... because ancestors are rare.

The recent/last great out of Africa population is one of those rare ancestor populations. Most lineages are dead ends.

We don't know much about them. We don't know which bones are theirs, or where they lived before dispersal. We don't know if they had been a distinct population for long... or a recent admixture homogenized before dispersal.