The short version of the reason: They said there is a half assed way of doing it and then there is the correct way of doing it which will allow stress testing of the actual game architecture. If they half ass it then it'll be out in December. If not, it'll be delayed. CR decided to delay it.
Long version: They could emulate their physics in Cry engine and put out a dog fighting module for the community to enjoy. This will allow for balance testing, ship design testing, control testing, user end load testing, etc. Or they can finish their actual persistent world node structure and test on that which would allow for all the above but would also allow for server side optimization testing, server load testing, and basically all the important bits. First they had decided to go with the emulation option but then CR decided to finish the server structure and test on that.