The Southern Contingent can give an alibi for that, (which is suspicious in itself)
I've no idea why it wasn't shown - there hasn't been a hint of it.
Not being ready seems a little unlikely, considering it was announced fairly close to it.
Maybe DNA/IMGlobal had an objection - I saw someone had asked Karl Urban if he was part of it at a Seattle con, and he said he'd only be part of an officially sanctioned short.
He seemed to be doing his own thing (his shooting down the sequel pushing from Karl with the 'no script' thing)
Anyway, no-one who knows has said anything, so it's all speculation.