One of the good replies to the poster!
Most of human history looks like this. Waves of migrations. First some nomad group settles or conquers the area, they cease to be nomadic, but after some time, another group of nomads conquers them and the cycle begins anew. Of course it's an oversimplification, because in time, settled peoples became so powerful that they were able to push the nomads back, but the areas close to the steppe are basically this, just look at history of Iran for example. In case of both Iran and Russia the nomadic raids were only stopped after the adoption of gunpowder. The thing is, that the Turks migrating to Anatolia weren't just a band of evil nomads wanting to conquer everything they saw, and then they just went and killed everybody they saw. No, by the time of the supposed "conquest", some of the Turks were already inside Anatolia, working as mercenaries for the Romans. The only reason why the takeover of the Anatolian peninsula was possible was that the Romans were busy fighting themselves, and there was nobody to guard the frontier. So in this political void, local warlords, often the Turks themselves, just stepped in and took control. Of course the Seljuk army was a factor, too, but people make it up to be some "evil barbarian Turk empire who took everything". If you look at history of Rome and the Caliphate, there are almost the same, but the Caliphate was basically the Rome on steroids. While it took Rome many centuries to fully dominate the Medditereanian, The Arabs were able to do the same in much, much shorter ammount of time. And the Turks were to the Arabs what Germanics were to Western Romans. People tend to have this wrong idea about how the "barbarian conquest" actually happened. Just like Germanics were already inside the empire, often settling there with their whole tribes, or serving in the military, Turks did the same in the caliphate. As nomads, they were interested in settling somewhere, and were able to do so. Just like Germanics, they served as mercenaries, and, sooner than later, acted as a warlords and controlled some pieces of the falling Empire (Caliphate). In the end, again, just like "barbaric" Germanics, the Turks ended up as the rulers and inheritors of their empire. So in a way, the Turkish takeover of Anatolia after the Manzikert was somehow similar to the fall of the western half of Rome, the difference was that the Turks didn't share the religion of the Romans, but then again, the early Turks didn't care about the religion, as they treated their fellow Muslims in the same way they did everybody else.