why was there no standardized set like the Romans used?

FWIW, the lorica segmentata wasn't necessarily universal across the Roman Empire, particularly in the later Empire due in part to the constitutio Antoniniana of 212, and then the 'Crisis of the Third Century' - a period of great political upheaval, civil war and plagues which wracked the Roman Empire from roughly the 230s onwards. While the lorica segmentata appears to have been standardised across the Legions at one point, by the Second Century, a great number of 'Romans' were Peregrini, essentially client tribes who had been allowed to migrate into Roman territory in return for providing Auxillia forces. These forces, many of whom were Germanic, tended to use their own weapons and equipment. In 212, the constitutio Antoniniana granted full citizenship to the Peregrini which negated the privilege that the Legions had of being truly Roman, and thus entitled to the status of better equipment. The Late Empire - from the 300s - also becomes increasingly reliant on Foederati. Like Peregrini these are 'Barbarian' tribes who are essentially contracted wholesale to provide Rome's military presence. Alaric, famous for sacking Rome in 410, for example, was one such Foederatus and actually led his men against Rome after the Empire's consistant failure to provide payment to him and his army. As entire mercenary armies rather than actual Legions, these Foederati use their own cultural weapons and armour.

The archaeological record also suggests that the use of Lorica Segmentata declines during the Third Century, as the level of upheaval, infighting and civil wars suggest that the Roman Legions were struggling to maintain basic manpower requirements, let alone provide full armour to all its recruits.

As to the Medieval period, the main reason that such armour drops out of use is sheer cost. One of the most overlooked and most important factors of the Imperial machine that disappears with the fracturing of the Roman Empire is the vast Imperial trade and logisitics network that underpins it. It is this network that allows regional specifications to occur; Rome is able to function as a vast industrial slum because its grain is supplied from North Africa. Other regions specialise in cash crops; olive oil or wine for example. The collapsing Empire is wracked by recession upon recession, and the collapse of this political and logistical network. This is accompanied by a wide population collapse, perhaps due to the series of wars, plagues, and famines which occur across the Empire. Peoples within the Empire are often forced to fall back on pre-Roman 'tribal' modes of government and very localised agrarian economies in order to survive. Cash crops and large scale industry essentially disappear as the vast majority of the population falls back on agriculture; this too means that the standing army of the Romans goes too.

That isn't to say that Roman strategies are completely forgotten however. Alfredian tactics for defending England against the Vikings are based in part on what Luttwack calls the 'defence in depth' strategy employed by the Romans on the Rhine frontier, with a mobile response designed to meet an aggressor in the field rather than hard defences to stop them crossing the border at all. Æthelflæd of Mercia consciously designs a series of fortifications to defend the Mercia-Danelaw border on the Roman model, with disparate garrisons supplied through a central logistics network. Such undertakings were highly expensive, and acheivable only when successor kingdoms acheived a relatively high level of political and territorial control and the logistical, economic and manpower potential that accompanied this.