This is a social definition. What society via its law making procedures wants for a definition shall be that definition. All other things are irrelevant.
** Manufacture definition**
If you have ever made your own tobacco or cannabis, this in itself is labor intensive. Making your own cannabis oil or resin even more so. Making alcohol is laborious and needs a great deal of care, whether brewing or distilling. Whereas poppy seed pods make really good tea
with virtually no effort. Quat is a leaf from eastern africa which is chewed like raw spinach- no preparation save harvesting- and it is soporific and habit inducing, causes gum and throat cancer etc. Trust me I’ve done all five.
Dangerousness
There is someting called a therapeutic ratio- that is the amount of drug that is effective versus the amount of drug which will kill or cause irreversible harm. All drugs of abuse pale beside Lithium Carbonate and Digoxin as far as danger in easily achieved overdose goes. Additionally, 30-60 acetaminophen (paracetamol) tabs will almost certainly result in major liver damage or death if it reaches the liver in concentration. Even large quantities of water will kill!
Many ‘Therapeutic’ drugs are more dangerous than any drug of abuse in normal dosage. I am pretty sure from experience (professional and voluntary) that long term use of marijuana is less dangerous than long term use of, say, major tranquillizers.
Altering Mental State
Anyone who thinks that nicotine does not alter one’s mental state has never really smoked- wake up at seven, mental state is awful, smoke cigarette, mental state feels good. 
What wqe are left with is a social definition. For instance: in Canada and much of Europe, cannabis is being officially recgnized as having medical use. Across most of Europe heroin (as diamorphine), Morphine Sulfate, Codeine etc. are all easily prescribable for pain relief as necessary. In Islamic east Africa and parts of Arabia, alcohol is illegal and Quat is legal. In Britain, alcohol is legal as is quat. I believe that quat is illegal and alcohol has been legal since the thirties in the States. :eek: Of course before that alcohol was treated as a drug- prohibition.
In Victorian England, Cannabis and Opium were easily available from any pharmacist without prescription.
Magic Mushrooms were, until recently, legal in Britain so long as no processing had taken place.
Until 2002 cannabis in the UK was seen as a serious drug of abuse and many police forces went out of their way to prosecute end users. Then it was dropped a class and Cannabis possession was treated far more leniently. Then the newspapers started making a fuss about ‘strong cannabis’ (as if this was new- cannabis oil and sensemilla have been around since the seventies- and that is at least as strong as skunk. Now there is a proposition to make mere possession an arrestable offence again.
To sum up, drugs of abuse are decided on by society without any real rationale behind the decision. Any formalistic rationale would fail and be unenforcible. What tends to happen is that the drug of choice of an underclass or the majority will be criminalized and the drug of choice of the powerful will resist such classification- it’s easier to pass laws that offend only the weak or a minority.