Why has Islamic Banking and Finance failed to deliver the promised socio-economic benefits?

The answer to this can be arguable. Some people might claim that it has actually brought positive socio-economic benefits but I tend to agree with you, the benefits that WERE promised havent been witnessed. We didn't really see a "financial revolution" because of Islamic Banking & Finance that many were expecting.

One reason is that there is still debate whether the so called present Islamic Banking & Finance is really Islamic or just a re-branding of the old banking system. Many religious scholars argue that present Islamic Banking Systems aren't Islamic at all (For e.g. Dr Israr Ahmed (founder of Tanzeem-e-Islami), Mufti Samiullah Khan (Ustad of Mufti Taqi Usmani) etc.Secondly, the system isn't perfect yet and there are a lot of short comings

Thirdly, the islamic banking hasn't been adopted in numbers that people were expecting. Compared to traditional banking and finance, the percentage share of Islamic banks is very low, hence you can't really expect there to be broad socio-economic benefits