According to :

It is simply because the crap they sell at Wal-Mart is not GI grade
(AKA 100 MPH duct tape). I had some I used on dive gear that held up
for many years and regularly used under water.
You can still buy it at some military type stores but it costs over
$10 a roll.

While perhaps not _quite_ Mil-Spec, you can get a very close equivalent
at other places. Lee Valley carries it for example.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.