He entered his plea at crown court so it's probably because one or more of the offences were indictable (can only be tried at crown court). Only offences which are summary only or triable either way can be heard at magistrates court.
Had it been just for sentencing, the plea would have been heard at magistrates court and the case "referred to crown court for sentencing". That's not what the article says happened, so that means it was not taken to crown court just for sentencing.
Edit: Just looked it up - "causing risk of serious damage to human welfare/national security" is section 3ZA of Computer Misuse Act 1990, which was added by Serious Crime Act 2015, and is indictable only, therefore cannot be heard at a magistrates court, hence straight to crown court for plea hearing.