Short answer: Two words – mens rea.
Long answer:
Well, most med-mal doesn’t lead to death, at least in a way sufficiently proximate enough that the actus reus of homicide has been accomplished. And to have attempt liability, the usual liability where one doesn’t succeed in pulling off the actus reus, you have to show the intention to perform the bad act, which is not typically present in med-mal. I suppose you could argue it hastens one’s demise, but I seem to recall a brief aside in my crim law class about old common law rules about how long one could linger before excluding the possibility of a murder charge. (And so the wikipedia agrees! See this article.)
But suppose we limit the discussion to med-mal where death ensues promptly. Murder requires malice aforethought (which means something entirely different from the lay term “malice”). Malice aforethought is any one of the following: acting or failing to undertake a legally required act with (1) the intention to cause death, (2) the intention to cause serious bodily injury, (3) gross recklessness endangering human life (depraved indifference), or (4) so acting or failing to act during the commission of a felony (felony murder). The degrees of murder are distinguished by the presence or absence of premeditation.
Obviously, of the four possible sources of malice aforethought, assuming this is real deal med-mal and not sadism under the guise of medical treatment, option (3) is the only one that could potentially give rise to murder liability in a medical treatment scenario. Now, I don’t doubt that there have been successful prosecutions with this theory, especially with respect to quacks and dangerous pseudo-medical treatments. However, there is a continuum of fault ranging from malice aforethought’s “gross recklessness” to ordinary “recklessness” to criminal negligence through to ordinary negligence. Ordinary negligence is often defined as not taking cost-effective measures to avoid the occurrence of an accident (the famous BPL Test or Hand Test of Carroll Towing). Gross recklessness endangering human life requires quite a bit more than just this.
So, murder is out. Manslaughter, like murder, divides into two classes. Voluntary manslaughter, which issues out of provocation, imperfect self-defense, and diminished capacity. Again, none of these really apply to med-mal. Involuntary manslaughter again comes in two conceptual flavors: constructive malice and criminal negligence. Constructive malice occurs where the intention to kill or cause injury is absent, but the intention to commit a crime or violate a safety statute intended to protect human life is present. Thus the bad intention to violate the law is construed as an intention to cause harm. We punish it more leniently because although we want a serious deterrent attaching to socially harmful activities, we recognize that the true (unconstrued, so to speak) mens rea is nowhere near that of an active intention to cause the death. And again, constructive malice usually doesn’t apply to med-mal.
But criminal negligence might. I mentioned the continuum of fault above and noted that some practitioners have been convicted on a murder-two count for gross recklessness. Likewise, some practitioners have been convicted for criminally negligent manslaughter.
Ultimately, it comes down to an actually quite refined system of penalizing varying degrees of recklessness and negligence. Unfortunately, while we have a pretty good definition of ordinary old negligence (B<PL under the Hand Test, or failing to undertake a safety precaution whose cost is less than the expected loss of injuries that the precaution would otherwise avert, where the defendant owed a duty of care to the victim), we do not have such precise definitions for the higher values of fault. Instead we leave it up to the finder of fact, using evocative terms as “depraved indifference” or “gross deviations for the ordinary standards of prudence applicable to the circumstances.” If we do not criminalize medical malpractice in the main, it is because we do not between typical medical malpractice escalates beyond the fault associated with ordinary civil negligence.