Because Ursula never appears in Elton's video diary. If she had, then Elton's story would be believable. Without an appearance in the video, Elton could be unreliable. Delusional. Crazy. Maybe he's not, but the possibility exists. Many will latch onto that.
And that brings me back to my point of why? There's still no evidence. The only reason to latch on to it is because you don't like the episode and it's still easier just to say it's a bad episode than to say it's not part of the same continuity because it's all a delusion.
The unreliable narrator is often in very good stories. Fight Club or the Murder of Roger Ackroyd are examples. In neither story does the audience have to determine an unreliable narrator simply because they don't like it nor would creating the unreliable narrator change whether or not you like it. The same goes for an ambiguous unreliable narrator (of sorts) in Inception with its ambiguous ending. Even there, whether the narrator is reliable is hinted at in a way that isn't mere speculation.
I see nothing but speculation here. You could also speculate that all of Doctor Who is in the mind of an autistic kid, but I don't see why you would.