First, I should point out that I am not a professional historian and so I don't have any specialty on this matter, but I do feel like I can give you a decent answer. To my knowledge, the Seljuk armies had an easier time maneuvering around the Anatolian terrain than their Byzantine enemies precisely because of the fact that they were mounted. The fact that the Seljuk's were also more of a tribal army then a professional one also partially helps explain the success of the Seljuk's, as many soldiers were used to living off of the land and many in fact still retained many of their tribal ways. The terrain was not in favor for the Byzantines because they had more infantry then the Seljuk's. I believe that the Byzantines also had more equipment and more of their soldiers wore more armor then the Seljuk's, Which meant that the Byzantine army was in general far less mobile than the Seljuk's. When you also take into account the fact that the Seljuk's were very skilled horse archers, then you can sort of start to see why the Seljuk's were able to expand into Anatolia the way they did.

But, when you say defeat, what do you exactly mean by that? Are you talking about the overall general defeat the Byzantines suffered during the Byzantine Seljuk wars when large parts of Anatolia had been conquered by the Seljuk's? or are you talking about the Battle of Manzikert specifically, the important battle that allowed the Seljuk's to expand into Anatolia in the first place? Well from what I know, the Seljuk's could not have expanded into Anatolia without the incredible victory that was achieved at Manzikert. This was because the Seljuk's had managed to not only destroy the Byzantine army but also capture the Byzantine Emperor himself. This left a power vacuum in the Byzantine empire which prevented the empire from organizing a cohesive defense against the Seljuk's, allowing them to rapidly expand into Anatolia. The fact that the Seljuk's were also more of a mounted tribal army than a regular army kind of played into their hands, as it was very difficult for the Byzantines, with their less mobile army, to march into Anatolia and just expel the Seljuk's. The fact that the Seljuk's also avoided pitched battle and used hit run tactics, to great effectiveness, with their horse archers also made it far more difficult for the Byzantines to defeat the Seljuk's. Also, and it's important I note this, the Byzantines made a terrible mistake when they decided to expand out of Anatolia and deeper into the middle east. I believe it was Basil II himself who specifically warned against capturing Aleppo because it would be extremely difficult and expensive to hold the city. Yet the Byzantines decided to expand out of Anatolia, which meant that the Anatolian terrains natural defenses were not as useful as before. Of course, the Byzantine expansion resulted in the Byzantines needing to protect their new found territory, and the battle of Manzikert was the result of the Byzantines defending the new territory they acquired, and as I already explained, that battle led to the Seljuk's quickly expanding into Anatolia. When the Mongol invasion happened, more Turkish tribes fled into Anatolia, which meant that the Turks had become more entrenched in Anatolia then before. I believe that most evidence supports the idea that Osman I, the founder of the Ottoman empire, the empire which captured Constantinople and put an end to the Byzantine empire, was the son of Ertugrul, who himself was a member of a tribe that had fled its homeland in Central Asia in order to escape the Mongols. This is kind of ironic when you consider that some Byzantines viewed the mongol invasion of Muslim lands as a good thing, as one of the tribes that escaped the Mongol invasions contained the founders of the dynasty that would conquer Constantinople and end the Byzantine empire itself.