Because no one wants to get stuck. The lack of liquidity means that a huge buy (say 10 BTC) would only momentarily drive up the price. Then if that person for any reason wanted to trade those coins, the probability they could do so profitably would be very low given current market depth, general willingness to pay, and the people willing to sell below the price at which the person who bought 10 BTC paid.
Increasing the number of exchanges would increase liquidity, signal a revitalizing market, and lower the risk associated with taking a substantial position.