I'm not sure if I worded this correctly. To my understanding (and I am VERY bad with Roman history) the late Roman period with the East and West Empires saw many "barbarian" tribes use longer swords which are "kinda" a precursor to medieval long swords. So these weapons were around during the Roman's time (for simplicity I'm talking about the two empires before the Western empire fell, so not talking about medieval Byzantium). I understand the Romans were very organised and had a set way of doing things and their short Roman swords worked well, but was the long sword ever implement? Why wasn't it more widely used in the empires yet in the dark ages it became the norm?
As I understand it you are asking two questions here; why did Romans not use long swords like their neighbours in the later Imperial period and why did long swords become commonly used after this period?
The answer two the first question is that actually the Roman military did use long swords of a similar length to those used amongst 'Barbarians' such as Gaulish, British and Germanic peoples. For the period of time about which you are asking, the shorter swords which we call the Gladius had fallen out of use in favour of longer swords which we know as the Spatha. This change happened from roughly the mid 2nd century AD. Before this, in the Earlier Imperial period we broadly identify the shorter gladius as being the weapon of the Legions: citizens of the Empire who fought on foot with a large shield in dense formation and we identify the spatha as the weapon of Cavalry as well as Auxiliaries (Non-citizens who may be from beyond the Empire's borders). Why shorter swords went out of use is a bit beyond the scope of this answer so I wont go into it.
The spatha appears to have derived from "celtic" or La Tene type weapons around the 1st century AD as the gladius got shorter and shorter, perhaps necessitating a long sword for non-legionaries.
Essentially, the Romans did in fact use long swords and it appears that they did in fact take inspiration from the weaponry of 'Barabarian' peoples from Northern Europe.
As an interesting aside, though there is a pervading idea that the Romans were in many ways technologically superior to their neighbours, when it came to sword making, in the earlier part of the Imperial period, this wasn't actually true. A metallurgical study by Lang (1988) shows that British La Tene type swords were typically of higher quality than Roman swords at the time of the conquest of Britain. It is only around the 3rd century that we start to see construction quality go up in Roman swords.
Bishop & Coulston. 2006. Roman Military equipment from the Punic Wars to the fall of Rome. Oxford. Oxbow
Lang, J. 1988. The metallography of some Roman swords. Britannia, Vol. 19, Pp. 199-215
Edit: To clarify, by "long sword" I mean a single-handed weapon with a long blade in comparison to shorter weapons such as the Gladius. To many people (myself included) the term "longsword" (all one word) is the name of a much later medieval weapon designed for use in two hands