Odie: Cybertron returned to Alpha Centauri after the events of "The Ultimate Doom," -- hence why all those crazy global storms that were tearing Earth apart stopped. I don't know how big the Maximals' expeditionary fleet was... we didn't really see any other expeditionary vessels besides the Axalon and the ship that Botanica served on. The only fleet we did see was actually a military fleet (日本の自衛隊みたいかなぁ*) -- although they didn't last very long against Unicron, heh.

It's possible that all travel to the Milky Way may have been forbidden, hence locating a tiny speck like Earth within an entire galaxy would be rather difficult. It would be very time and resource consuming to scour the Milky Way to find Earth... much like if we were to try scouring Earth's oceans in search of Atlantis. It would be far easier to steal the Golden Disk and learn of Earth's exact location.
*
like Japan's self defence force