Because it does matter how it happened. Most legal systems, as well as most people's moral judgements on the issue, are going look at the totality of the circumstances.
Someone going 3x the speed limit is creating a more dangerous situation and should face a higher penalty than someone who was, say, following all the rules but sneezed and accidentally hit a cyclist.
It doesn't feel reasonable to say someone who gets wasted and swerves into a cyclist has the same degree of responsibility as someone who fails to see the victim. Both bad, but drunky made worse choices and introduced more danger. Similarly, speeders have less time to react, lower levels of control, and have exponentially increased the impact forces in a crash.
Obviously, intentionally hitting someone and killing them is the worst but personal choices that increase risk raise culpability.