It was called the Dark Ages for a reason. The vast majority of a kingdom's dwindling manpower had to be dedicated to producing food, so keeping a large, trained standing army was mostly impossible. Your knights would be well trained, heavily armored, and probably fighting from horseback. You have to rely on peasant levies for infantry. Good luck getting those rabble to fight in Roman formations.
And God help you if you were attacked during harvest season.
(Disclaimer: I am not a historian, and learned most of my history from playing historical war games on my PC. 
Its no longer called the Dark Age, and just how far "civilisation" fell has been revised. It is true that food was scarcer, and especially early medieval ages relied more on peasant levies, but most armies would have been able to count on small amounts of men-at-arms provided as part of a knights military obligation. A larger problem was that armies came together to fight, unlike Roman legions knights didn't tend to train and live with each other, thus military formations and tactics could not rely high levels of co-ordination and discipline.
During the later eras of Medieval times weren't there a lot of more professional mercenary groups? Couldn't some of those have adopted similar tactics?
They could, but this age was dominated by heavy cavalry initially, and the counter to that is not shield walls and short swords, but pikes. As pikes become larger wielding a shield as well is not possible. Some armies did use mixed formations where some soldiers carried shields and others pikes as a compromise.
