User blog comment:HELO/Le-WHY-athan/@comment-2249774-20120908054510/@comment-4950455-20120908064214

Or maybe they didn't think it was really that important - it doesn't really matter how the Leviathans lost, just that they did.

I think it's made clear that, at least in the modern day, the Leviathans are a powerful force, but can't singlehandedly defeat the Reapers (and, to judge by the nature of their War Asset, still aren't that interested in trying). That's what we really need to know about their capabilities - the exact details of what went down a billion years ago aren't that important.