I haven't documented the choice of R6RS anywhere, so here is as good of place as any.

The main reason was familiarity. R6RS is the Scheme I know best. I learned on R5RS and moved on the R6 when it was the new hotness. I vaguely was aware of R7 (I have friends that were on the standards committee), but didn't really know much more than that it had happened.

There's a technical reason for choosing R6RS as well though. To me the defining characteristic of R6RS is the library system. In my mind, this has a pretty clear mapping to WebAssembly modules. Scheme libraries import some set of identifiers and export another set. WebAssembly modules are the same way, so I basically set it up so that Scheme exports and imports map directly only WebAssembly exports and imports.

I'd definitely be open to supporting other Scheme versions in the future.