They wont.
Directbet "uses" betfair back odds, so they will never offer any new markets .
Why they use only Betfair? Can't they add new markets from other betting sites? Isn't it technically possible?
Directbet is an automated system, a software (that uses Betfair exchange and game API) , if you are a good programmer you can develop the same betting site.
(perhaps it would take 1 year but if you have more developer then half a year is enough (to clone directbet)
If the program is ready then you can sit back and almost do nothing. You have to only add a customer service. (1-2 person)
Why on earth would you add more markets (that cannot be automated) to an automated system that needs only 1 person to maintain , if you were still billionaire (Directbet).