Don't forget network effects - betfair have the market, and there are NO successful competing exchanges, though plenty have tried (and are currently trying). Liquidity is everything - if you want exchange betting, you use the exchange that everyone else is using. Lower commissions or smarter software are meaningless if there's no-one there to match your bets with.
It's the same reason why there's only one Ebay. That's a company that makes loads of money, yet no serious competitor has survived (remember QXL?). Network effects again.
Betfair have won this battle - don't expect that to change just because a bunch of city boys with connections are trying to copy the idea.