Another good example of this are the Steam sales, especially the big ones (summer and christmas sales render the store completely unusable for the first day or so)
It's "good enough" for the week of the sale, upgrading all the servers for a week of super heavy traffic just to see them idle the rest of the year isn't a great business plan
There are several firms that offer computing capacity online, at much more granular terms than "the rest of the year". It seems these "cloud" computing services are common topics of discussion here on HN.
The calculus has to be how much it would cost to temporarily bring in, run, maintain e.g. AWS boxes versus how much revenue is left on the table by the slow store. I presume Blizzard has run those numbers and decided it wasn't worth it.
