Tough to say. It might be an Avengers: Endgame situation where the announcement is a spoiler, so who knows. The last prequel news was months ago, just to tease that we won't be seeing anything until sometime in 2020.
Finances play a role in large-scale con events like this. It costs a buttload of money to fly and put up a cast, especially if they're flying internationally. It may not seem like they care too much about that, given how much the show cost, but the show is wrapped, and the press tour is largely over. HBO has other screaming babies to feed, like the prequel (but also , all of their many many shows).
It's still a little early to say, but if they weren't willing to have one last year before the final season, I'd bet they'd do it again for a prequel we know nothing about, unless some major kiloton knowledge bombs drop before then.
(disclosure, this is all speculation of what I know of entertainment. As far as GOTs info goes, I know nothing, Jon Snow)