Because BQ is great for the ETL/data warehouse/BI use case but is terrible for online applications. I tried using BQ as the backing store for an online analytics application back in late 2019, and it was so much worse than using Clickhouse/Druid/Pinot for the same use case. IDK how much that has changed since, but I'm not too terribly surprised that it isn't higher.

I'm not on GCP but the one I would like to try is Spanner or Cloud Spanner.

I think more scalable systems will continue to gain market share. It will be interesting to see if PlanetScale, CockroachDb or some other actually becomes a big player.

Bigquery seems like a tool for very large but static datasets. I also had a hard time figuring out the pricing so other than some test queries I moved on to other solutions.