It's only at the end of this BBC report http://www.bbc.co.uk/news/uk-england-london-40059351 that we find out the why:
'Smith grew up in Devon and only moved to London to attend university three months before leaving his homemade bomb on the Tube.
An only child who lived with his mother from a young age, he has a form of autism that impairs social communications but not his intelligence.
An able student, Smith also displayed a persistent fascination with Islamic terrorism and mass murders.
Friends were shown violent propaganda by so-called Islamic State and Smith created YouTube videos of himself using computer games to re-enact infamous real-life mass killings.
In a shopping list of the components for his bomb, he had written: "And keep this a secret between me and Allah #InspireTheBelievers."'
Yes, it's Islam again.