Because then companies that happen to profit from your death have an incentive to convince you that you should die, using manipulative advertising and marketing.
Because then people who are highly depressed might kill themselves with assistance from a doctor, when an alternate route could cure them.
Because then the doctor might forge evidence showing that you wanted to be killed and it could be hard for the police to track down. Maybe they only use the attack against certain high-reward targets.
Because then somebody who is impulsive might agree to be killed when he would've regretted it later if he were still alive.
I tend to be in favor of this for people who legitimately want to die, but there's something to be said for the simplicity of "The autopsy showed poison, so we have a murder case." I could be convinced to make it illegal if somebody found some really plausible attacks that could be used against me; and could be convinced to make it legal if we found and required a scientific test with a legally-enforcable chain-of-custody that proved I was in my right mind when I asked to be killed.