2) A perception as "natural", together with the naturalistic fallacy. The black plague and cancer are "natural" too, but we have no qualms eradicating them; yet for some reason, people romanticize death, especially when thinking primarily about it applying to other people rather than themselves.
3) Related to that naturalistic fallacy, aging afflicts a less sympathetic group. Given a disease that strikes down the "young and healthy", or the affliction hidden behind such thoughtless and abominable phrases as "it was their time" or "natural causes", the former will tend to attract more funding.
4) In a word, spirituality, whether spoken of or otherwise.
5) Too many crackpots through history have tried to peddle life extension, creating guilt by association.
2) regulation strongly constrains funding by steering it towards drug discovery.
3) people don't like being told they're doing it wrong, even when it is absolutely true.
4) the real goal of most researchers is to catalog metabolism, not to produce any way of treating aging.
It costs two billion dollars or more to push through one drug candidate for a given desired effect. Current estimates suggest you could implement the entire SENS program of rejuvenation treatments in mice for that amount of money over 10 years. Those are technologies with potentially unbound gains on healthy life span; not just slowing down aging, but actually reversing its causes. Periodic repair has no limit on the degree to which it can extend life, and is far more effective for old people than merely slowing aging. Given the options here I know what I'd rather see the research community focusing on, but getting more R&D to focus on repair over metabolic tinkering is an ongoing process of persuasion and disruption.
You might look at senescent cell clearance for an example of a SENS technology that is slowly breaking into mainstream research notice. Oh so slowly. It's almost painful to watch so much time and effort being directed to old-style mine the natural world drug discovery for marginal treatments unlikely to move the needle when we're in the midst of a biotechnology revolution and so much more is possible.
[1]: http://sens.org/research/introduction-to-sens-research
[2]: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3836174/