Alliston was acquitted of manslaughter. Given it was a jury trial and there is little contemporaneous reporting of the trial, details of the manslaughter charge or his defence to it (as opposed to the conviction and sentencing which was widely reported), it's difficult to know why this was. But the bar for proving manslaughter (whether on the basis of gross negligence or an intentional unlawful act) is quite a lot higher than that driving was "dangerous" (and which in turn is a higher bar than that it was "careless") - this is why it's rarely prosecuted for cars.
It does seem reasonably likely that in a counterfactual world where a very similar fact pattern had occurred but in a car, he would have been convicted of causing death by dangerous driving - and his resulting custodial sentence would likely have been meaningfully longer (the offence he was actually convicted of carries a maximum sentence of only 2 years vs causing death by dangerous driving which can now carry up to a life sentence - although at the time of his offence the maximum sentence was 14 years).