Why there is not a newer driver for Windows 8.1? The newer drivers are for Win 7/10 only.
The latest I could find for Win 8.1 is Crimson ReLive Edition 17.7.1. (dated from July 2017)
This from a previous thread concerning Windows 8.1 support for RX5xx series card:
ray_m Dec 31, 2017 9:45 AM (in response to utopiia)
Driver support for Windows 8.1 has been discontinued.
AMD Halts Support for Windows 8.1 with Consumer Radeon drivers