Because it is justified, indeed commanded, by Allah in the Quran...
A young teen boy, indoctrinated in a Taliban-run madrassah, detonated his bomb in the midst of Jama Masjid prayers, killing 55 and injuring another 80 or more.
Such attacks are so commonplace and seemingly mundane that they don't make top stories of the BBC News website nor even the top 10 read stories. People just shrug their shoulders and repeat ad-infinitum the familiar chorus that “he couldn't have been a Muslim” to do this.
The bomber was indeed a Muslim boy, apparently 15 years old, indoctrinated by Muslims and it was precisely his Islamic indoctrination that programmed him to commit this atrocity, which has become so commonplace that no-one pays serious attention to it anymore.
The Muslim worshippers blown to pieces in this mosque-attack were apparently from a tribe working against the Taliban in the area:
The Kokikhel tribe has recently formed a “Lashkar” (Anti-Taliban militia) against growing Islamic militancy in their area. The Taliban frequently attack the tribal Lashkar, who are supported by government.
So the Taliban used the Friday prayer congregation in the biggest mosque in town as an opportunity to inflict maximum casualties to members of a tribe that oppose them.
The Taliban found nothing in Islamic teachings to stop them from committing it. Didn't they think that Islam is against Terror as apologists and the naive parrot day in day out, in particular after the latest terrorist atrocity done precisely in the name of Islam?
Of course not! The Taliban are learned scholars of Islam trained in Madrassahs, which train students exclusively in the Quran, Ahadiths and Islamic Law. The very name Taliban means “students of Islamic learning”.
The Taliban know Islam well; that Islam counsels terror as its modus operandi. Terrorising enemies of Allah is commanded by the Quran (8:60):
Against them make ready your strength to the utmost of your power, including steeds of war, to strike terror into (the hearts of) the enemies, of Allah and your enemies
And this modus operandi was employed by the early Muslims under Muhammad’s command, and has continued to bear fruits to this day – thanks to the fact that it is a command by Allah, enshrined in his Quran, to remain in effect for all eternity.
"I have been made victorious with terror" says the Prophet of Islam, and indeed he had.
The Taliban are following the dictates of the Quran to fight for establishing the dictatorship of Islamic laws:
And fight them on until there is no more tumult or oppression, and there prevail justice and faith in Allah altogether and everywhere. (8:39)
All law and rule not based on Sharia are considered “tumult and oppression” by Islamic definition and fighting is commanded forever against such governance, even against nominal Muslims who fail to completely implement the Sharia. Such Muslims are cursed in the Quran as munafiqoon or hypocrites, the so-called moderate Muslims who fail to fully implement Sharia or go for Jihad against non-believers. The Quran asks Allah to "slay the Munafiqoon", strange seeing as Allah is supposedly the author of the Quran:
[Munafiqoon 63:4] And when you see them, their appearance would please you; and when they speak, you would listen carefully to their speech; like wooden blocks propped against the wall; they assume every cry to be against them; they are the enemy, so beware of them; may Allah slay them! Where are they reverting!
So the supremacist teachings of Islam do in fact inspire the kind of terror that the Taliban perpetrate, and there is nothing strange about atrocities, such as blowing up of Mosques, full of praying Muslims, who are not fully submitting to Islamic sharia and thus becoming munafiqoon in the eyes of the fully devoted and indoctrinated Muslims (Taliban).
So, it’s easy to understand now why the Taliban can send an indoctrinated 15-year-old boy to blow up a Jama Masjid.
As devout Muslims continues to wreak havoc upon innocent people, it is high time for the nominal (moderate) Muslims with civilized conscience, who have reservations / doubts about Islam, to study Islamic texts carefully to understand what Islam truly stands for and consider whether it is worth embracing it as their faith, their guide to life. It’s certainly better to be an infidel and free human than being a hypocrite, which is safer for all of humanity too.