I’m pretty sure that Knuth used assembly language (and his own hardware design) in “The Art of Computer Programming” because he wanted to teach the fundamentals, with “less saturated fat” (as he would say) than other systems had to offer. A high-level language would have obscured many of the points he was making, especially regarding optimal algorithms.

By the way, after reading our Plain English Programming manual, Knuth wrote me to say, “This certainly is an elegant proof of concept, and one of the nicest self-describing systems I've ever seen.” It seems that, while we don’t agree on everything, Knuth and I are kindred spirits when it comes to teaching the fundamentals in the simplest possible terms. More about Plain English can be found here:

The Osmosian Order of Plain English Programmers Welcomes You

And if you think I’m making this up, here is the letter Knuth sent, written in his own hand on the cover of the manual I sent for his review: