My reasoning being that 'Orcs are capable of counter-spelling the Tower'. Very large numbers of people objected in the strongest of terms to that idea, and, from what I can tell are still trying to, despite BoneyM directly saying that it wasn't hugely harder to counterspell than the base spell.
There were definitely claims that only this dragon was swole enough to manage it, whether because of Hysh type advantage over Ulgu, its vast experience, its resistance to damage, or whatever.
I'm not sure we were already going to be keeping Thorek in reserve, and we certainly hadn't committed to sending Mathilde to run the tower when the time came. I do think the degree of piling on that I had to endure before BoneyM responded and confirmed the core of my point, that the Tower could be counterspelled, was unnecessary and very unpleasant.
On a slightly different subject,
@BoneyM, do we know if it's possible for a counter-speller to damage or temporarily disable the tower, analogously to how a miscast is induced in human spellcasting? I know that on the tabletop this is what happens to bound spells like this that 'miscast', but we all know that the tabletop is hardly a reliable guide to what should happen with this grade of magic...