Another thing to remember is the message sent in Regeneration. It would have undoubtedly had information about the future. As well as future technological information it would have had something like, "In 2373, the Borg Queen will launch a mission in which a Sphere travels to Earth in a Cube. Attacked by Federation Forces, the Sphere leaves the Cube and uses a temporal rift to travel back in time to 2063. Once there, you will launch an attack on the Phoenix's hanger, but will be quickly destroyed. You must beam aboard the Enterprise with some drones and attempt to assimilate the ship. Once your attempt to create an interplexing beacon fails, attempt to destroy the Pheonix after it is launched. This attempt will also fail, and the drones aboard the Enterprise will all be destroyed. However, some drones on the surface that withstood the crash of the Sphere's wreckage will be able to send this message at a later date."
Now, if the Queen in 24th Century gets that message and attempts to do something different, it is possible that something will go wrong. For example, in Regeneration, we see they recover two drones, let's call them 1 of 2 and 2 of 2, or 1 and 2, respectively.
If the Queen decides to do something other than what the message said, she could make it impossible for 1 and 2 to send the message. Let's say that instead of sending the Sphere to Earth inside the Cube, she sends it by herself. In that case, without the Cube to protect the sphere, it's possible the fleet could have destroyed it, thus ending the mission and creating a problem - if 1 and 2 are killed, they won't exist in Regeneration to send the message, and thus the mission would never have been launched.
Or if the Queen decides to send the sphere back in time first and then travel to Earth. Then something could happen en route which would damage the sphere or kill 1 and 2. Again, the message is not sent.
In short, the Queen knew of only one way for her to have received that message, and that was to do exactly what the message described. And that's why she did it that way.