We don't need a "leading exchange". We need exchanges where all customers can get money and Bitcoins in and out with no delays and no nonsense. All exchanges where assets flow easily in and out will converge on price, because any difference will cause arbitrage transactions to pull them together.
1. One of the oldest exchanges 2. Located outside the EU / US 3. Ease of deposits / withdrawals 4. Availability of other crypto-currencies 5. Right now, the largest BTC volumes. 6. Never been hacked or DDoSed.