Iirc, it's because of Petrarch, who was frustrated with the lack of written sources from the period. The term was also used by Voltaire for the same reason. It wasn't that dark, at least the people living during it didn't seem to think so. It's important to remember that all of those "invasions" tended to be more incremental, but by putting hard bounds on it, in our minds it comes across as German apocalypse. If you look at the sources, you find that it was even more a case of migration and attempted assimilation, hence why Germans were commanding Roman armies against other Teutonic forces. Not to stir up shit, but you can look at the current migrant issue around the world and see how people describe a rather prosaic event, with headlines like "Invasion" being popular a popular way to frame the scenario, it wasn't that different back then. No, it wasn't some collective trauma that is being referred to by "Dark Ages", just a current lack of sources. Those sources are being discovered and analyzed now and an interesting picture of an ingenious adaptation to circumstance is being developed.