Why do closed-end funds trade at a 10% discount to their underlying assets?

 Economist:  No one knows, but they are becoming less popular.  

Persistent discounts violate one of the fundamental assumptions of efficient financial markets: the law of one price. This holds that two identical assets should converge in price, and that long-term differences must reflect intervention or friction. ... As far back as 1949, Benjamin Graham, an author and investor, called the discount “an expensive monument erected to the inertia and stupidity of stockholders”.

In the early 1950s, just after Graham penned his attack, closed-end funds held assets worth almost 70% of those in mutual funds. ... Now closed-end funds are outgunned not just by mutual funds but by exchange-traded ones, too. Among the three categories, [closed-end funds] hold just 1% of total assets.

OK, but if the few remaining closed-end funds represent an arbitrage opportunity, why can't we buy them, sell the underlying assets, and earn 10%?  

Turns out that someone else thought of it first: activists are targeting a range of closed-end funds, including ten run by BlackRock.