Nobody really wanted to integrate RISC-V support anywhere while the ISA and ABI was unstable. That only really got finalized last year, so I am not surprised that it took this long.
This actually seems like the most reasonable reason to me. It wasn't stable, there was nobody (planning) on making RISC-V devices on which you'd want to run Android, and compiler support was far behind. We're in a very different place now, I would have made that same call (though perhaps would have given more reasoning to reduce speculation).
Is this more about the ABI? I couldn't find incompatible userspace ISA changes since many years (things sounded to have settled down since a while even in 2016: https://riscv.org/wp-content/uploads/2016/06/riscv-spec-v2.1...). The RISC-V Linux port was also merged to mainline in 2017 and presumably been living under the stern binary & ABI compatibility rules there.
Since ISA collections affect ABI in RISC-V, getting an agreement of the baseline for RISC-V ISA for Linux-based systems was extremely difficult. On top of that, the ABI stabilization work couldn't begin until the required ISA extensions for Linux were locked in. So it was all a mess.
