Murdo, I think the distinction may be in the minds of jurors: they think "that could have been me driving and accidentally killing someone", whereas they know perfectly well they'd never handle a gun in the first place.
This isn't irrational: it involves an underlying assumption that cars are used by almost everyone for transport, and are generally vital to the running of society, and given the volume of use, momentary inattention by a lot of people is inevitable Guns on the other hand have only purpose, killing something, and any carelessness at all will be looked on more severely.
I'm not talking particularly about this case - sending texts while you drive is lunacy - but I think it's fair that the justice system distinguishes between cars and guns.