There's a hypothesis that people with bipolar disorder show 'accelerated aging' and this could account for part of the difference in life expectancy. But I think it is hard to disentangle. First you have suicide as already mentioned, then there is the fact that people with bipolar are more often smokers, drug addicts and/or alcoholics. Then there's a factor from being poorer and more isolated than the general population. (Isolation also reduces life expectancy._ Then there's the long term effects of medications on weight gain, metabolic syndrome, diabetes as well as lack of activity or participation in healthy activities.

That order is not meant to indicate which are the most important just that there are so many factors to disentangle that any attempt to do so would get a result that depended heavily on the statistical methods used.

Basically people with bipolar have crummier lives than the rest of the population by and large (not true for everyone by any means) and this is seen in life expectancy. I think it is the size of the difference that is so staggering and depressing.