Because once this material had begun to spread, these proto-gospels, then it was out there and couldn't be retrieved. Kind of like a picture being put on the internet: if you wish for that picture to be spread in a changed version instead of the one that has been uploaded, then you'll have to act quickly! Once it's out there, it's out there.
Maybe an unfortunate example. The Christians against Dinosaurs hoax of a few years ago featured a "demonstration against dinosaurs" photo which had been 'shopped so that protestors' placards denounced dinosaurs. The source photo was a real demonstration against a parking fee increase in a small UK town. The original photo was on the web, the event had been covered by the BBC and local media (also on the web), the genuine photo came up in Google's search by image facility if you cropped the fake version a bit, the original was actually spotted as the original early on (but the guy who tweeted it neglected to give a source, so people could reasonably wonder whether he'd shopped the CAD photo, rather than CAD shopping the photo he'd spotted)...
Perhaps more timely, since the autograph is coming up for auction in December, there's the fake English translation of the German language Einstein-Gutkind ("God") letter that was widely circulated in mass-media circles, thanks largely to the Guardian
pimping it. Einstein calling Bible stories childish was edited in, and his narrow qualification on the specific kind of Judaism he found primitive, etc. was edited out. Some prominent German media back-translated the Guardian
English version to arrive at a new text in the original language (and of course the Germans disagreed with each other on the specific wording). This farce persisted though the 2012 sale, despite high resolution photos of the autograph becoming easy to locate. Even for this upcoming sale, and even though Christie's itself is using and directing inquirers to correct transcriptions, some prominent media are still peddling the made-up version.
The lesson I draw from these examples is that you raise a very
interesting question - how can anybody possibly get away with "improving" a document that's already out there? But there is no question that somebody can get away with it, because Christians agaist Dinosaurs and the Guardian
newspaper have done just that, despite 21st Century technology to establish the truth instantly - in principle.