In-universe, because the people living on Elysium were greedy and wanted to hoard the technology for themselves.
Out of universe, it's because the beds aren't intended to be real. According to the film's director Neil Blomkpamp, they're just a metaphor for medical poverty:
So, on Elysium you can imagine that if you went totally speculative
sci-fi in the year 2154, the film becomes different, then you’re
making a film about speculative ideas of what society might be like
and what will happen is they’re no longer rich. Now it’s more like
Star Trek, like there’s something else and the link between, ‘Oh,
they’re rich people with pools’ has been lost.
So the medpods weren’t there for science fiction reasons, they were there just to represent that the poverty stricken person from El
Salvador could go into parts of LA to get better medical attention or
medical care for their kids. Medical aid is one of the things the
First World has and longevity if you just look at the stats. So it was
more a metaphor tool, but you are right in the sense about … like,
well how far do they go, at what point is it just immortality? I think
they probably live a few hundred years and then they die in the
construct of the film.