Romanian language is issued from Roman speeches that were more or less widely spoken along Danube river, in Moesia for instance, not from the Dacian cores, while it's debated if we're talking of the southern basin (that can be tied up with thraco-roman) or northern basin (that is daco-roman) or probably both (at least IMO).
Even if Rome withdrawed, don't forget that the Danube basin was in constant contact with Romania (as were the non-romance peoples as well). It certainly helped to preserve proto-Romanian.
We know that roman speeches were used in the Northern Byzantium by the Early Middle-Ages while we don't know at which extent, and because of the lack of litterary sources, we'll have a really hard time to know.
Romance-speaking people most probably did the same than others ones and mixed with Avars, Slavs, etc. while keeping their language.
Eventually, what won the decision south of Danube was what was the ecclesiastical language : greek for Constantinople, slavonic for the Slavic churches, which played a huge role in their respective importance.
While most of southern eastern Romance was divided as isolated by distinct slavic entities (basically, being at the margin of two or more cultural-politic entities), northern Danubian basin was kind of a political vaacum, where steppe confederation dwelled but never really settled up or forming political-cultural entities. It probably allowed Romance-speaking peoples to prosper, by removing an incitative for linguistical shift.
And having it becoming a chancery and religious language by the XVth managed to stabilize it.
That said, pre-XIX Romanian was particularly influenced by slavic and hungarian, and had to share large portions of its linguistical territory with hungarian, bulgarian, russian, serb, etc. There, the usual suspect is the rise of nationalism which lead, as in other countries, to a politically-driven cultural hegemony.