I think there are multiple reasons. The ones we all leap to are financial. The need for wage slaves. Even if someone can't work, they fit in the capitalist machine- creating the need for pharmaceuticals, providing jobs for carers etc. No doubt, religions still have power too.
As for allowing suicide for all though- that wouldn't be a popular choice for parents. I think people have this concept that it's only our governments keeping us here against our will. It isn't just them- it's our families, friends and especially our parents. They simply don't want to lose their children via suicide.
They certainly don't want the government to be allowing them access to lethal substances. Imagine the rioting in the streets the moment the first few 18-20 year olds take their lives. (If it was introduced.) How many votes do you suppose they'd win by saying: 'We're going to introduce programmes whereby your children can gain access to lethal drugs!' I think to a greater extent- our governments are simply listening to popular opinion when they don't allow suicide for all.
The argument doesn't stand at all when it comes to the aging population. While they may generate jobs- they must be a massive drain on resources. It would surely suit our governments to start culling them. And- why stop there? Why not the ill and homeless too? I imagine it would benefit them to get rid of chunks of society. So- why don't they? Because those people are still other people's relations! And- even if it was supposedly that person's 'free' choice- we likely wouldn't like it as a society if our governments started killing us off!
It also may like look like the inhumane option ultimately. Why bother paying for a heroin addicts methadone for months on end, providing them with housing and benefits when a single euthanasia shot would be cheaper? I think governments realise how that would look so- they try to at least appear to do the moral thing and (supposedly) support people to live.
I imagine there would also be many investigations as to why a person ended up in the position they were in- where suicide seemed so appealing. Were there failings in society that ought to have taken care of them? So- was it actually their 'free choice' to die or, were they left with little alternative because no one would help them?
I think allowing assisted suicide would be holding up a massive magnifying glass on our societies failings. As it is, people can be neglected and forgotten about till they die. If they start being killed off young- more might start paying attention.
