Number of variants to a large extent. Risc-V has a relatively few number of implementations that are about the same in terms of how they implement things. Aarch64 has a comparably huge number of implementations that have different pipeline descriptions, different extension APIs supported, etc. Aarch64 includes descriptors for everything from a low-end Cortex A53 designed back in 2012 to a a high-end Apple M2 that was released this year.