Unlike many other known major demic and linguistic expansions, however, the historical forces that allowed the Slavic migrants to utterly swamp the pre-existing residents and bring about language shift are largely unknown and are not even the subject of much speculation, despite the fact that this was happening at the dawn of the historic era and some of the people who witnessed it must have been literate, and despite the fact that the ancestors of these people still live in these places today.
Was there a technology or cultural trait that made this dominance possible? Had famine or plague or raids by migration era raiding tribes so weakened the existing residents that the land was all but vacant? If these weren't factors, what were? Investigation of the whys of Slavic migration is a task that should be on the "to do" list of anthropologists and historians as a major unsolved problem.
Wikipedia describes much of what we know about the early Slavs, but this information doesn't itself provide obvious answers to the big question of "why the Slavs were so awesome".
The leading narrative seems to focus on a power vacuum created by the fall of the Roman and Hun Empires, and by the disruption of society caused by raiding tribes (mostly Germanic) in the migration period who raided territory and then left when it was duly pillaged, rather acting like the Slavs and raiding and then ruling the land that they raided. The Slav had recently rebelled from being overpopulated Iron Age farmers subjugated to barbarians, which presumably made them willing to fight for more land and made them familiar with the raiding lifestyle that they did not fully adopt.
(Factual error corrected later on the same day posted in response to a comment.)