Currently the genesis block is compiled into the software. If this is still the case at launch, then it won't be possible to release the final signed binaries before launch. This might cause operational problems such as a rush to download the final binaries, reduced time to test them, etc.

If the mainnet genesis block information were in a separate file, then that could be released on October 28 even though the software can have been signed and tested well in advance. (Note that if it's generated after the software is signed then it can't be authenticated by a hash in the software, but it could be signed by the same mechanism as the software.)