why a regional and hitherto virtually unknown group
I think the assessment that they were "virtually unknown" is wrong in the first place.
They were known, but under different names than "Slavs".
This is also explanation why "people from nowhere" could expand so fast - they were simply not "from nowhere".
And also the role of Hunnic invasion was important for creating prospects for Slavic expansion - the Huns "cleared the path" for the Slavs.
They were also not a "regional" group - they lived in quite a large territory most probably.
Mosquito said:
In the 10 and 11th century Poles were fighting with Roman Empire, Czech Kingdom, Kiev Rus, Denmark, Prussians and Elbe Slavs but they were rather far from territory of nowaday Romania.
and still you are not nationalist?
What was nationalistic in Mosquito's post where he simply stated facts that Poland in the 10th and 11th centuries fought against the HRE, the Czech Kingdom, Kievan Rus, Prussians, Polabian Slavs and Denmark, but not against Vlachs? These are facts and facts cannot be nationalistic.