Lisp is conspicuously absent while even COBOL gets a fair evaluation.
Can anybody account for this?
I would say the very first paragraph of the introduction on page 8 sets a foundation that pretty much excludes Lisp on the spot:
A quick search will reveal that the word 'Embedded' shows up in every chapter for a total of 31 times. A strong hint that the embedded theme was the main target for Ada.
COBOL is, unlike Lisp, suitable for embedded applications, not at least due to its ability to generate extremely compact code. Of course it will fail utterly when looking at other requirements (see below).
Lisp may be great to handle complex data relations in highly variable data sets, but I doubt it is first choice for embedded applications. Especially not back in the 1970s when even high-end control systems had at best a few dozen KiB of code space.
When looking at the 'general requirements' noted on page 12, it can easy be seen that Lisp fails in essentially all categories named.
Looking at those criteria helps to understand why several other languages are also not included. FORTH, for example, a language often praised for its great performance in embedded designs, holds only barely better than Lisp with a single check mark at efficiency.
Looking at the list of languages evaluated does suggest that a good number (including COBOL) may have not been added because of them fitting well, but rather due them being already used in DoD related applications before.
have come across references to Lisp later being used for US-government related work
I'm not entirely convinced that such an anecdotal story makes a good argument. While it misses any placement on the continuum of designated applications, I'm pretty sure it's not about an embedded one :)