The fact is that we need more than "just enough" differently focused market makers to support a healthy ecosystem within the exchange. Some are focused on horse racing, some on football. Some do only tennis, some might be really good at cricket. Golf, motorsports, baseball, basketball, ... the list goes on. Then come the sport specific variations: some are better at pre-play, some at in-play. The sports and market matrix is pretty damn big.
And like in any exchange, being a market maker comes with contractual obligations. They need to provide two-sided quotes, good quote coverage, sufficient liquidity and tight enough spreads. All of which are requirements for a good retail user experience. In return we guarantee them (among other things) reliable event data updates, capacity to sustain an agreed order rate, technical support and an increasing amount of back-office functions.
The API will be opened once we are confident the exchange and its ecosystem can sustain the flux.