Two things:

In that video, the crossbowmen outnumber the Dacians 3:1, and the Dacians have to charge into that fire without any supporting missile fire or cavalry screening. They're going to get mown down pretty handily in that case.

Second, crossbows weren't exactly common in the Eastern Roman army; a few archers may have had them, yes, but for the most part the Justinian-era army still relied on the recurve bow.

Source:

Elton, Hugh, "Army and Battle in the Age of Justinian," in Erdkamp, Paul, A Companion to the Roman Army.