Iirc WASM was initially explicitly designed to be translate-able to Asm.JS (that in turn translated to JS), basically they continued the way Asm.JS had retained

Iirc WASM was initially explicitly designed to be translate-able to Asm.JS (that in turn translated to JS), basically they continued the way Asm.JS had retained backwards compatiblity with JS to either force other browsers to play along in implementing the standard or just look bad in benchmarks. (I think they've relaxed on back-compat now but it was a tool for getting everyone on board and once that had happened it was easier to break it). This is why WASM kind-of-is a stack machine. Basically the opcode sequences should be possible to easily re-translate back into a syntax tree for a back-compat JS generator. This is also why you have these block/loop branches.. it basically is translateable to labelled while loops and if blocks.