There are a lot of factors at play. But I think the biggest one is lack of expertise in the community. For postgress isn't that hard to hire a professional with 10 years of real world experience.

Besides that you need to keep in mind that scylla isn't a silver bullet. There are some workloads that it can't handle very well. Where I work we tried to use it in workload with high read and writes and it had trouble to keep up.

In the end we switched back to postgres + redis because it was cheaper.