Some things are better off left unknown.
With something like Transformers it's going to be hard to weave together a satisfying, and logical, explanation.
Wolverine was hugely popular for decades, and a large part of that was due to the fact that he didn't know his history.
It finally got explained, and a lot of people weren't happy with it. Their assumptions were cooler than the actual story. His Origin series sold well, but was definitely not universally liked at all.
Another good example: Star Wars. Decades of mystery of the past. Awesome sounding events like 'The Clone Wars'. What did it mean? Dunno, but it sounded awesome.
Then look at the prequels we got. A whiny little kid.
How many of the fans of the original trilogy were happy with the prequels, and their explanation of events before 'A New Hope'?
We're all these events as awesome as what you'd pictured and imagined in your head for years?
Sometimes a little mystery goes a long way.
Proud Member Of The 'Resurrect Bumper' Club.
...A Club Of One.