Because Scala 3, as great as it is, has fractured the community and caused a lot of churn.
Twice, just in my recent job history, have I worked for companies that were using Scala 2 for a long time but are now deciding to develop new projects in Java and/or Kotlin instead.
I think that is less about Scala 3, and more that the companies were not committed to the use of Scala, and would eventually move to some other flavor of the month language, such as Go or Rust.
The transition to Scala 3 at my current job has not been an issue. Scala 3 is mostly the same language, with some nice new features which are optional. Our old projects are still on Scala 2, as there isn't a huge benefit from upgrading.
The main downside in upgrading was library support, but we are now 2+ years since the release of Scala 3, the issue is mostly solved unless you depend on unmaintained libraries.
By contrast, over the last 8 years I've migrated a number of small micro services up from every version of Scala since 2.10 with no problems, maybe some minor dependency hell with some of them. I've even upgraded a simple project to Scala 3 with no code changes.
It depends on what libraries you use. The big hurdles tend to involve macros and Spark.
Twice, just in my recent job history, have I worked for companies that were using Scala 2 for a long time but are now deciding to develop new projects in Java and/or Kotlin instead.