Leaving aside the technical issues I think a bit of fear of the media has a role in this. A dreadful and spectacular accident causes several peoples death in the most appalling manner . even so in English law manslaughter cases can be very hard to prove and i think that is the heart of the matter. The reasonable doubt which smart lawyers can introduce into many scenarios puts the conviction at risk but if he had been charged with a lesser offence then the mail and Express and the rest of the gutter press would be baying for the blood of the CPS for being weak and selling out bereaved families
While Uk has a decent legal system it still has many flaws (how much money you have is still too important) as we often see when a large company causes the death of employees or passengers or member of the general public no one ends up getting charged but if you operate fairground machinery-or ina recent case a bouncy castle heaven help you.