I'm grateful for the author's "other options that were considered" section with eight alternatives to S7. But Embeddable Common Lisp (ECL) is the only one listed that he doesn't write a few sentences about, to contrast its pros and cons versus S7. I'd love to hear more.
OP: it was a two factor decision.
The first is that it seemed at the time that Scheme was going to be easier to work with, and that has proven true. It's a lot easier to do music theory hacking and integrate with Max using a lisp 1. Being able to seamlessly change whatever the first form is (function, hash-table, list, vector) with no change to the code has turned out to be really really nice in my work.
The second was that when I started I knew little about Schemes and Lisps and it seemed when going through the docs like ECL at that time was going to be a bigger lift to get going.
Solid reason, thank you. Yes, Lisp 1 for the win in many cases.
Now... there's also the wonderful Fennel possibility which distracts me when I'm bikeshedding. ;-)
Yes, Fennel is a great distraction (see my other post)!
clang and ECL both seem interesting, as long as LGPL license is ok. They both are complex to embed though - neither are well tuned to be embedded in other applications. (I've been exploring on MacOS). Julia could be interesting too under same questions (and a better license for commercial products) but similarly has a fair bit of glue code needed - especially if you want to restrict (say) filesystem access.
Oh right, that was a thing too. LGPL would have complicated some plans. Forgot that details.