They don't get to choose. They don't own the copyright on Redis, Salvatore does. He is their employee now but if they try to pull something like this I suspect he won't be for long.
Based on this interview from 2016 Salvatore
agrees with the issues of BSD and cloud vendors so he may be a lot more amenable to a license change than you suspect: https://venturebeat.com/2016/06/19/redis-creator/
>But now, for the first time, GPL could be interesting again because of the cloud vendors. Because with BSD, cloud vendors are able to extract a lot of value from an open-source project, to the point of making it very hard for the project’s initial creators to make a business out of it. Let’s call it the “AWS problem.” The AWS problem, technically, could be enough in some way to create problems for the whole open-source ecosystem. Now people know that if they start an open-source project and put a lot of effort into it, they could be marginalized by AWS. That could mean that GPL would return again as the primary license for open-source software in the future.
>Redis’ permissive BSD open source license allows them to do so legally, but this must be changed.
Doesn't seem like having Redis core contain enough useful functionality and staying BSD lines up with their vision.