They weren't particularly on the menu for government contracts. An extreme example/analogy is "Why don't all AR15 patterns use a JP Silent Capture Spring?"
It simply wasn't the appetite for customers to use them since the original buffers have been used for (what was then) 20+ years.
When HK designed them, they also had to deal with the fact that they must sell them, not include them free of charge.
How convenient would it have been for commercial 91s to have come with them, though? I wish PTR would have done the same.
PS: These buffers are also a bit wider than standard ones. If HK wanted to sell the early units, they would have had to retroactively modify much of the A2 stocks issued. Not ideal.