Because it is and always has been a relatively common name in the Muslim world, it's

  an Arabic form of the Aramaic given name Yohanan of John the Baptist in Islam, who is considered a prophet. 

~ https://en.wikipedia.org/wiki/Yahya_(name)

Another, perhaps more interesting question, is why is now Marxist libertarian, formerly Trotskyist Revolutionary Communist Brendan O'Neill following the lead of Daniel Ben-David of The Jewish Chronicle and "just asking questions" about something that both admit

   there is no definite correlation between the increased popularity of the name and support for the terrorist also bearing the name

~ https://www.thejc.com/news/uk/yahya-soars-up-baby-name-leade...

This seems like a typical Telegraph non story.