The phrase "history is written by the winners" is commonly uttered these days, which goes to show how many people repeat things without thinking about them, since a moment's thought would reveal that it is incorrect. Plenty of history has been written by losers, for any and every meaning of the word "loser". Many histories of the Civil War were written by the defeated Confederates, and the influence of those histories continues even today. Defeated Germans likewise wrote memoirs of their World War II experiences -- memoirs which have not always been believed by the victors, but which in some respects have been and deserve to be.
One might make a closer approach to the truth by saying that history is not written by people who lost their lives. But even that ignores posthumous memoirs, such as Anne Frank's or Rommel's. History is not written by people who were dead at the time of writing -- okay, finally a real truth.
Besides losers of wars and losers of lives, there is also the everyday sort of loserdom, where someone distrusts his own instincts and knowledge and dooms himself to losing. Plenty of those people have written history, too; they're often the same people who repeat phrases like "history is written by the winners".
Now, this isn't to deny that historical facts can be suppressed; they often have been. But suppression works much better in the short term than in the long term, and history is about the long-term view. Even totalitarian countries sometimes change their minds about what their official doctrine is to be (as in Khrushchev's Secret Speech); and even things written under a totalitarian system often leak out the truth in their details despite a superficial show of compliance with official doctrine. And, of course, totalitarian countries have enemies who, even if totalitarian themselves, eagerly exploit leaks and information from defectors. The competing narratives create confusion for historians to sort out, but they have decades to sort it out. Although some details forever remain obscure, we don't need to know every detail to learn the main lessons of the past.
The freedom to ignore those lessons seems to be the appeal of the phrase. By slandering past histories as mere winners' propaganda, it gives a broad license to revisionism. It also excuses errors in that revisionism: if you're a "winner", you can make whatever errors you and your fellow winners want; that being the standard, who is to say you should do any better? Or if you're a loser, you are excused in making whatever errors the winners tell you to make. This appeals to people whose connection to reality was always quite tenuous, and who are glad to be relieved of the obligation to adhere to it.