^^^^1 word: Tax.
With a few exceptions, most illegal drugs are relatively cheap (or free) to produce or obtain. If they were legal and the gov. taxed them like cigs (an aside: Have you seen the price of cigs in the UK?) most peeps wouldn't be buying them off the shelf. So instead they try to socially condition us (with the threat of a criminal record and punnishment if need be) to make do with their MORE destructive, taxable drugs (Booze & cigs).
@ Rawkus..... Try all the drugs 'you' listed as "should be legal", then come back and tell us if you STILL think it'd be a good idea for them to be legal (preferably sticking a few vid clips on YouTube).