One reason might be that Black Ops 2 comes out the week before Wii U is released, and I'm sure every store will be having a midnight launch for it. It's very rare that they have more than one midnight launch per month, let alone two of them just a few days apart. There's a lot more that goes into midnight launches than just paying employees extra to be there late.
EDIT: Went to EB today, they actually did a midnight launch for ACIII (so did basically every other store), and will do one for Halo 4 and obviously Black Ops 2. I didn't, stupidly, ask about a midnight launch for the Wii U but I'll give them a call during the day.
Not to mention later that week GameStop will have a Black Friday sale. So that's four midnight launches within three weeks, very tiring for the employees I assume.
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