For the same reason as everything else done or available: because there are willing consumers, and who are you [or me] to impede their choice? The abuse to be prevented [as an example, nothing personal] is if you want to commit your suicide while owing me, say, $100 [or having any other unsatisfied contractual obligation, not necessarily to me] - and without paying your debt or making the provisions to pay it in your will. Thus the need for the due process and for a judge to sign off.
See my previous reference to hit men / contract killers. Euthanasia is killing someone as a matter of convenience for others...no different than me having a creditor offed so they stop pestering me. Convenience and demand do not by themselves, legitimize a practice.
So you see murder as a consumer driven issue?