Because the system is designed to fuck you, the retail investor–the poor bastard that doesn’t get to buy the stock until it is in the secondary market.
Meanwhile, the IPO takes place in the primary market: The banks “determine” the price of the stock (and charge Twitter a separate fee) for doing so. In reality, the banks set this price as low as they can get away with, so they can dump it in the secondary market (poor bastard individual investors who buy on excitement alone) for massive profit.
Essentially, initial public offerings (IPOs) are a means for Big Money into scamming you for more big money. Here, Twitter’s stock price doubled in the secondary market, which means Twitter got fucked out of $10 billion in investment money that it would have received in the secondary market, but which instead went into the pockets of the investment banking sector. Bravo, Wall Street.