> [T]here's no real reason why ALL companies need to have their stocks compatible
I think it would be way harder to trade a basket of stocks (e.g. pairs trading, going long one and short the other) if you had to worry about mismatched settlement dates across the different stocks; it would be like trading spot against a one-day forward.
You've just defined exactly what high frequency traders do to make money. They balance all stock exchanges in order to make tiny profits on the stock differences.
They don't, there are hundreds of exchanges and plenty of competitors for listings - CSDs & clearing are a pinch point.
But in some ways exchanges also function as a natural monopoly; especially for primary issuance. e.g. look at AMEX's IPO slate compared to the big two, where the market is right now - it's insignificant, and they are the #3 exchange in the US. People want to list to make money, and that means going where the liquidity is until there's a really good reason not to, like with NASDAQ's move to electronic trading in the '70s.
