There are a variety of variables that affect Bitcoin pricing on the exchanges. Some are:
- Market size
- Exchange volume
- Price of entry
Market size: Relatively speaking, the market for Bitcoins is small. In April of 2013 it was about 1.2 billion USD, and a few days later dropped to below 750 million USD. That's a small market cap, which means, among other things, that there's less consensus on the price to the BTC. This is the capitalization for all mined bitcoins and includes coins that have been lost, so the real value is something smaller. Each exchange is a subset of the total market, so those markets are even smaller, which allows for greater variation.
Exchange Volume: For all the coins that have been mined, the quotes are only from online exchanges, which are a small set of the total coins that have been mined. If only a quarter or less are in play, then the swings can be pretty dramatic. Since the volume is limited, and people don't take full advantage or arbitrage, different prices can and will exist on the different exchanges.
This happens with foreign currency exchanges too. However, with professional traders, billions of dollars, and serious automated trading the differences are in fractions of a percent.
Price of entry: It's relatively cheap to enter the BTC market and cheap to trade. Furthermore, if you mined BTC when it was trading for fractions of a dollar, or even $30/BTC, there's no issue unloading it. With a low price of entry people are less serious about how the trade their BTC.
Fundamentally, BTC is a small, highly speculative, irrational market. Each exchange is a small, highly speculative, irrational market. That's why the spread is so great.
As a crude example:
Consider two towns in the medieval period separated by twenty miles. They both have markets and people sell apples in both markets. In one market people really like apples. They pay 2 coins for the apples. In another market they aren't as enthusiastic, they only pay 1.5 coins for the apples. The economic thing to do would be to buy the apples in the second market and sell them at the first. But it's hard. It's a twenty mile walk, and all your friends are at the other market. Plus, the price could change by time you get there. It's just easier to stick with your current market, even if you're not maximizing your return.