Probably because it's of a type that ack doesn't recognize. ack's searching behavior is
driven by filetype. If ack doesn't know what kind of file it is, ack ignores the file.
Use the -f switch to see a list of files that ack will search for you.
If you want ack to search files that it doesn't recognize, use the -a switch.
From the FAQ section:
Reported by @rjbs in the ack1 queue.