okay, lore drop time, just to everyone is on the same page here.
The autobots used the last space bridge to escape to earth, shutting it off just as they came through the other side. Meanwhile the decepticons were still in rush mode, and couldn't get to the bridge in time. So, unfortunately, they had to make their journey manually. The autobots hid on earth for a helluva long time, staying as low beneath the radar as possible and using up small amounts of the earth's energon reserves before the decepticons came. Then, well, the decpeticons came.
The autobots used the last space bridge to escape to earth, shutting it off just as they came through the other side. Meanwhile the decepticons were still in rush mode, and couldn't get to the bridge in time. So, unfortunately, they had to make their journey manually. The autobots hid on earth for a helluva long time, staying as low beneath the radar as possible and using up small amounts of the earth's energon reserves before the decepticons came. Then, well, the decpeticons came.