Closest I can come is the old "I want it to be more painful than that" movie villain logic (which we did see from Cersei with, uhh, whatsername from Dorne). But that seems like a hell of a stretch given the other issues here.

Regarding Tyrion and Qyburn, yeah, I'd been thinking: if there's one person whose JOB it is to anticipate things like Qyburn's new ballistas, it's Tyrion. So maybe he fucked up again.

The reason I can imagine them not anticipating the upgraded ballistas is because I remember the scene in season 1 where Tyrion gave the Starks the design for a saddle for Bran. It was just a simple bit of engineering but people looked at him like he was some kind of sorcerer.

For the upcoming episode, Tyrion's assignment to invent a thing that destroys ballistas.