It is the tiny half life and the fact that it is mostly a decay product of things having **very** much longer half lives.
There is a route via Bismuth capturing an alpha particle which is interesting, but I don't think the cross section works to get that to go directly as a chain reaction which is a pity.
If the cross sections were better, you could build a pile of bismuth, hit it with a few alphas and watch as the thing gradually ramped up a mixture of Alphas (that would actually be driving the reaction) and betas plus heat, longest half life in the main line of the resulting decay series would be ~30 minutes which would make it a cool play toy for the shed, but with a delayed alpha gain of 3 it does not work.
Back to the drawing board on that one....
Regards, Dan.