Why isn't Africa attracting Portfolio investment?

PSD Blog reports on the reasons for tepid portfolio investment interest in Africa:

Despite their recently good performance, Sub-Saharan African stock exchanges lose out because of their small size and very low liquidity. As he put it, "the New York Stock Exchange trades more before tea than all of Africa trades in a year." The glaring exception is the Johannesburg Stock Exchange (JSE), which is as large and popular as any emerging market stock exchange.
The bottom line - a stock exchange must have $50 billion in market capitalization and $10 billion in value traded to attract any interest from global emerging market funds. Of the 15 African exchanges, only South Africa hits either metric.