This week NASA announced that it had awarded Boeing the first Commercial Crew flight contract under the CCtCap program. However it is important to realize that while the contract has been awarded, as NASA started in the press release.
Determination of which company will fly its mission to the station first will be made at a later time.
So why were Boeing selected first? This is a matter of scheduling, the Boeing CST-100 module will be launched atop a ULA Atlas V rocket to ensure that they are able to launch by late 2017 they need to book the flight now to enter the processing flow. SpaceX has a shorter lead time for their rockets as they manufacture the whole system internally.
There is no denying that this is a significant step both for NASA and Boeing but as they stated there is not guarantee that just because they were awarded the first contract means they will actual fly the first contracted flight. This will be determined once future milestones in the CCtCap program have been completed and NASA can be confident that they can complete the flight when needed.