The main reason is because of the casting. The second reason is because of the set. The Barbican and the Palladium have very similar sized stages, and both their auditoriums are huge (though the latter has a larger capacity). They wanted to use the touring set for the Palladium, which I think is even smaller than the set they used at the Barbican, which I thought was ghastly anyway.
You kept on saying you thought the Palladium was too big (it isn't, stage wise), and that WoZ will continue for ever (it won't either). The key reason why SP is not transferring is that Paulo Szot isn't. You never mentioned that.
In the end, you were right in saying SP won;t go to the Palladium, thouugh I suspect that was just a wish of yours which turned into reality. Your reasoning wasn't right, so please don't think you are above us all now. You're not. When Wizard of Oz celebrates its 25th anniverary at the Palladium then you might be, but the chances of that happening are zilch. It's more likely that you'll get blown up in a twister and taken to some magical land where lions and scarecrows can speak. And that'll NEVER happen.