The purpose of a reserve is to be money available for spending when needed. And by money I mean cash or its nearest thing (hence the money markets). You cannot put this money into the stock market (I swear they have laws for this), since could you imagine going to the condo members and saying:
"So the markets took a bit of a tumble and we lost all your condo fees from the past 5 years. By the way, we need to make up that shortfall now, so expect an increase."
Reserve funds are to be used for emergencies, as such they need to be in low-risk investments. Since the market rewards based on the amount of risk you take (in theory), it is expected to make a poor return. And if it troubles you that it is wasted potential, think of the cost that would result if you didn't have money and the roof needed to be replaced. Instantly the value of all the units drop and if the board was somehow able to take out a loan I can promise you it would not be at a very happy interest rate.