Uganda, May 16 -- By the standards of emerging market investing, East Africa's institutional portfolios have long resembled a monoculture. Pension funds, insurance companies, endowments, and foundations have for decades planted the overwhelming majority of their capital in government securities.

It was rational because sovereign paper offered double-digit yields, near-zero default risk, regulatory comfort, and the kind of liquidity that keeps trustees sleeping soundly. When one asset class pays 18 percent and nothing else comes close, diversification feels less like prudence and more like sacrifice. For years, the trade-off made sense.

But that field's allure is thinning. Yields are compressing and as they fall, from 18 percent toward 1...