Mostly because the popular conception of a horde of Germanic invaders overrunning and destroying the Roman empire is wrong.

Also because "centuries of genocide and defeat" is rather overstating things. The Romans were unpleasant neighbours to be sure, who could react to a raid across the Rhine with a punetary expedition that would slaughter anyone they could find, but this was not on a large scale.

Instead, it is more likely that centuries of contact with Rome resulted in the Germanic speaking peoples to the north developing ever closer ties, getting access to ever more Roman trade goods including large quantities of Roman weapons, having generations of their people be experienced veterans of the Roman army in which they would serve, and developing larger scale, more powerful confederations themselves precisely to be able to resist Roman pressure more effectively. In many ways you could say that the Germanic peoples that would eventually come to dominate the former Western Roman Empire were formed in a reaction to the Roman Empire itself. The adaption went both ways, in fact: the later Roman army actually adopted various Germanic symbols and customs, such as their standards and their war-cry. (The barritus) This is a phenomon we see more often in history in such frontier regions, where the people living on the border start to resemble one another more and more over time.

This was also true when you look at their political ideologies and languages of power. These Germanic speaking peoples did not, for the most part, WANT to topple Rome. They wanted to be part of it. And not a subjugated part, but influential movers and shakers. They wanted Roman wealth and land and culture, they wanted to mimic Roman forms of rulership. They wanted to be the empire, not destroy it.

Speaking in purely military terms, the Germanic peoples were never at an advantage over the Romans. The Roman empire was larger than any one tribe, had many more soldiers, and those soldiers were (even in the late empire) on average better equipped, trained, and more professional. If it came to a battle, the Romans usually won. Usually.

So why did the (Western) Roman empire fall? Some historians would argue that it didn't, but that's just rephrasing the question. How did various Germanic speaking people come to rule the various lands that used to form the Western Roman empire?

That's an extremely complicated question with many different answers and disagreements. The short of the what happened would be that in the crisis of the 5th century, the Western empire gradually lost control of its provinces as they prioritised fighting civil wars and dealing with internal rivals over asserting control over their periphery, which eventually eroded the tax base of the empire to such a point that it couldn't really maintain an army and a government anymore, at which point a military strongman named Flavius Odoacer decided not to bother pretending anymore and had himself proclaimed king of Italy instead. (But theoretically still part of the Roman empire, subject to the Eastern Emperor)

There were episodes that could be termed as invasions, such as the attacks of the Huns or the Vandals taking over Spain and then North Africa. But a lot of the territory was lost to nominal Roman allies instead, armies or peoples (Historians argue over which term is correct) like the Franks or the Goths who were serving the Romans, but also pursuing their own agendas, and with central authority eroding started to just claim lands for themselves. And some areas were just straight up abandoned by the Romans, such as Britain and much of northern France. (Named now after the Franks who took over.)

There were also plenty of attempts by the central Roman government (or factions pretending thereto) to reassert their control, right up until the last few years before the traditional fall of the Western empire, when a combined Eastern and Western Roman attempt to re-conquer North Africa failed to Geiseric's fire-ships.

The why question is even more complicated than the what. Literally hundreds of reasons have been proposed in more or less serious academic works over the centuries, ranging from the sensible to the absurd, and many of them completely contradictory. (Linked list is from the 80s, we've no doubt had many more since then.) There are economic arguments, demographic, climatologic, all stressing long-term factors. There are those that stress political factors, often to do with legitimacy, the sharing of power and lack of stable successions. Some claim military factors, be it the Romans getting weaker or their enemies getting stronger. There are those that think it's more about incidental factors, mistakes made and battles lost and accidents of fate. And plenty of historians disagree about any and all of the above.

I go into a few broad interpretations in this older post here There's also a section in the FAQ that about more specific questions.