My guess is it's the license change. From Wikipedia:

In 2018, some modules for Redis adopted the SSPL. In 2024, the main Redis code switched to dual-licensed under the Redis Source Available License v2 and the Server Side Public License v1.

Valkey appears to be a Redis fork that was triggered by the license change, but since Valkey still uses the original BSD license, I'm not sure I'd favour it over Redis since the latter switched licences specifically to prevent abuse of the BSD license by parties like Amazon.