I can speculate on the WISC-V: it has five index scores (clusters) vs only four on the -IV, so that each one represents a smaller portion of general intelligence. The VSI and FRI put together are similar to the PRI, so it is more comparable to require two of them than only one.
As for why the achievement scores are no longer required, it has been speculated that too many 2e kids were being unintentionally excluded and that this was an effort to remediate that.