It is the fact you have to sell to realize the loss and most people doing so are likely to be looking for capital losses to offset gains in other areas, as it is tax season. Second Market only accepted accredited investors.

Accredited investor is a term defined by various countries' securities laws that delineates investors permitted to invest in certain types of higher risk investments including seed money, limited partnerships, hedge funds, private placements, and angel investor networks. The term generally includes wealthy individuals and organizations such as banks, insurance companies, significant charities, some corporations, endowments, and retirement plans.
In the United States, for an individual to be considered an accredited investor, they must have a net worth of at least one million US dollars, not including the value of their primary residence or have income at least $200,000 each year for the last two years (or $300,000 together with their spouse if married) and have the expectation to make the same amount this year."[1]
In Canada, the same prerequisites apply, however their net worth must be a minimum of one million dollars not including the value of their principal residence

-http://en.wikipedia.org/wiki/Accredited_investor