Where are you getting the "vastly larger installed base"? On popcon riscv64 is a lot higher than sh4: https://popcon.debian.org/stat/submission-1year.png

There may be more hardware out there with SH4 processors, like some NAS boxes, but are they using or contributing to Debian? And when I look at a list of hardware with SuperH processors, half of the links are broken links: https://elinux.org/Processors#SuperH

SuperH seems like a niche architecture mostly supported by a single vendor and available in a few devices mostly in the Japanese market, without a huge amount of use in the Debian community.

RISC-V may not yet have as much extant hardware available, but there's tons of active development, and the open licensing model means that a lot of companies are working on RISC-V chips, which will then flow into hardware.