Projects which take many years to have an effect (the impact of pre-school education on university admissions, say) will not interest investors.I have outlined my reasons for advocating tradability of the bonds here. It does appear, I'm pleased to say, that there are bodies interested in making a secondary market for SIBs.
When I first came up with the idea of Social Policy Bonds one of my colleagues told me that I was 20 years ahead of my time. He was almost right: it's been 25 years. This long pdf contains my first published paper on the subject and is dated 1988.