Not a mystery, all twelve are stored in the Arizona desert at 309th AMARG Davis Monthan AFB, covered in white spraylat protective coating. At least they were late last year when I saw them there.
If I recall correctly, the only formal reason given was 'technical difficulties during implementation of the contract'. That can mean a lot of different things...
I also recall a Pak Army statement that the deal was not cancelled.
Seems they are not freed up for onward sale to another customer or use by the US Marines either, but still stored. The cost of storage is paid for by the USA as far as I know.