Continuing the discussion from VLC Thing. A Poor Man's Sonos:
Because entrepreneurs realize taking risk often leads to the greatest potential rewards. Gold Rush. When the platform becomes stable, development will be much easier, and that means a lot more competition for the same functionality. Take a rules engine, hypothetically. It will be easier to write after seeing a few attempts (even without shared code), but in the meantime, someone could attempt to profit from one and have a head-start in the market. Plenty of effort and risk (ST could launch their own rules engine built into the product, etc.).
Now my answer is getting long, but I gotta add: Far too many vendors are on the sidelines waiting for HomeKit because they think that the Apple marketplace will make it easier to monetize and market. SmartThings may not be the ideal alternative, but it is better than doing nothing.