does he ever explain why the sounds sl and du are the ones that developed to be negative?
Well first, if you (or anyone) thinks I'm going to actually READ all of something like that, then you have me confused!

But, I would venture to guess that nowhere in there does he allege that these are the only two sounds that have, uh, slipped down to the dungeon. (And I'm sure also that he isn't saying that all words with these sounds are pejorative; look at slim and slender, for ex.)

I think it makes sense, when you think about it, that images/emotions would come to be associated with certain sounds--that's how we get onomatopoeia, after all. And it only takes a slightly larger stretch of the imagination to think of these same associations carrying over to similar words/sounds.

Interestingly enough, I was doing an unrelated search just a little while ago, and came across a post that not only reflects my own mindset, but may even have influenced it (too long ago to remember for sure, Sweetie), and was hoping to find an apt spot to quote it. Here 'tis:
What I am moving towards is the general *belief that what distinguishes us at our very core is our tendency to make patterns. This is true of language, true of visual sense, and surely true of the other ways we think, such as forming theories. We tend to adopt short-cuts (perception theory is littered with examples of how the brain ‘fools itself’) and discard material that doesn’t sit happily with our frame of reference – because ultimately it is not the veracity but the aesthetics of the pattern that counts to a key part of our imagination!

As a matter of fact I believe this is what John Keats means by “beauty is truth…”

http://wordsmith.org/board/showthreaded.pl?Cat=&Board=miscellany&Number=7585