I think its because it got repeated around a lot. And still does.
Also, I think people like to ascribe "semi-canonical" status to things where the on-screen creator was involved in some way. (Whether or not said writers said they were "canon.") For example: Jeri Taylor's Voyager books. Those were often declared "canon" by the fans back when they came out, even though, as I recall, they were contradicted by later episodes.
A lot of times such books can involve back-story that was created for the series but not shown on-screen. (Jeri Taylor's books used stuff she wrote for the series bible, for example.)
I think what people miss is that "canonicity" isn't about some sort of hidden knowledge the creators have about the past, its about the constraints that the writers have to be held to in order to tell a story in the future. So even if something was in Bob Orci's head, or the Voyager series bible, at one point, that doesn't make it canon until it shows up on the screen. Because they might change their minds or have a better idea later on.