It's a bit like the 'war on drugs'

The last thing drugs kingpins want is for them to be legalised,

Right now, they make 100% profit at whatever level they operate at.

Legalise them and they make less (because tax).

So, the people lobbying hardest to keep them illegal are those trading in them - it's much cheaper to lobby a few politcos with a few brown envelopes to push their agenda.