If I had to guess, it's because the WebAssembly Text Format is S-Expressions, similar to the target language, Scheme. Also, less tooling involved to build the binary, since the text format is pretty close to the binary format.
In practice though, it looks closer to C/ASM. The author even wrote comments in C. This is the first time I've looked at an actual program written in the wasm text format and it's pretty interesting to see how one would actually use it. Very neat.
I've definitely evolved my "style" as I've worked on it, the earliest code I was thinking in C and writing in WAT (hence the comment style), As I got more familiar I changed how I wrote it (adding the macro layer helped with that)
In practice though, it looks closer to C/ASM. The author even wrote comments in C. This is the first time I've looked at an actual program written in the wasm text format and it's pretty interesting to see how one would actually use it. Very neat.