Because the BetFair trading engine handles a huge number of small bets, they had to lower its cost per transaction.

Matt Carter (Director of the Advanced Technology Group at BetFair), interviewed by Silicon:

Matt Carter, Betfair director of architecture and advanced technology, explained to silicon.com customer trends are moving towards a high volume of small-value bets, which, in turn, produce small-value commissions. He said the only solution was to develop a system that could process exponentially more transactions for a lower cost than traditional betting engines. [Matt] Carter said: “It wasn’t a case of whether we needed to do this, we have to do it. It’s a question of survivability.”