Quoted from RobT:

Because it's priced too high.

Quoted from TheFamilyArcade:

It's all timing. And probably location to boot.
ACDC is in a window now where a lot of original buyers changed hands already (last year or two), current owners are happy, License expired and no desireable games currently shipping except for MMr. Not to mention a couple other situations where $ is tied up with no machines forthcoming so there's a perception of "no great games" being released. Prices increase during times like this, but it's doubtful they'll stay this way. There are a ton of ACDC out there. Some desireable games appear to be headed down the pike - or primed for release (AFM anyone?) - and around that time more people should also be tired of their METs, STs and even their Trons. I'd bet that ACDC is not a $6k game in that competitive climate. But where's the competition now for, to many, Sterns best game?
Today $6k may just be the benchmark (unless your Gweempose). Last month I sold mine to a local pinsider for less than I might have received otherwise in an open market ($5,600) because frankly an open market can be a pain and I wasn't in the mood. But that is another reference point, just like RobT's. Clear window with about the same value of mods as this one.