Why not the other 49 Muslim-majority countries?

There are fifty Muslim majority countries in the world. BBC's Wusat Ullah Khan has asked a pertinent question. Why Pakistan is the only 'jihad' laboratory in the world (See Baqi unchas kuyu nahee (Why not the other 49)?.

He has argued that Pakistan should not be a safe heaven for Muslim extremists because Pakistan has a large, well-organized and efficient army that can control its borders. However, currently extremists enjoy control of almost one-fourth of Pakistan's territory. Why?  

Khan then debunks another myth. He contends that if extremism is linked to the first Afghan war in the 1980s, then extremists should also be present in other Muslim-majority states bordering Afghanistan but they are not. Extremists are only in Pakistan. Why?

Moreover, Khan clarifies that most of the young people who came for jihad in 1980s are now in their 60s while the extremists now seen in Pakistan are in their teens or twenties so they are not from the earlier era. So earlier Afghan jihad explains little. 

So, at the end of his article, he has asked why Pakistan is the only one ?

I think Pakistan is the only one infested with extremism because (unfortunately) several factors came together here in Pakistan at the same time:

1. Pakistan is a state based on religious nationalism so nationalist feeling helped population accept Muslims coming from other countries in the 1980s and 2000s. Even now there is large reservoir of sympathy for Taliban in Pakistan.
2. Pakistani state or state agencies helped extremists previously and some of them continue to help them. So, these extremists never felt the consistent power of Pakistan army for a long time. 
3. The area under extremists control (Federally Administered Tribal Areas (FATA)) was never under direct Pakistan control. Despite its name, it was never really administered. These were lawless borderlands so people in this area never knew state. There was hardly any political, economic or social development in this area in the first fifty years of Pakistan life. Obviously, now Pakistan and its people are paying the price of this neglect.
4. The border with Afghanistan is hilly and difficult to monitor. With people of same ethnicity living on both sides of the border, border management is not easy. Drug trade further complicate the situation, presently huge incentives to people who want this area to remain unstable.