You can build Scala projects with Ant, Maven and Gradle, but for some reason, a language community often gravitates toward the language-specific tools.

I don't think it was a conscious decision to not support Gradle, etc, it's just that sbt had a few highly motivated maintainers. This meant that sbt became the easiest tool to use when starting a Scala project.

sbt's incremental compilation was also very important for a language with long compile times and still-developing IDE support.

It would be great if Scala had great Gradle support. Nowadays some large Scala users maintain Gradle support for Scala to a good level.

It's possible that back in 2008 Gradle did not seem like an obvious choice (assuming it is now; I have no experience with it.) And after a community has effectively standardised on a tool, switching to something else isn't done lightly.

The Scala community provides Gradle plugins for scala itself (through zinc), scoverage, scalastyle, scalafmt, wartremover, scala protobuf integration, and scala-aware intellij project generation from your build.gradle.

It works well.

When sbt came out, Gradle wasn't as popular, the choices were either Maven or Ant at the time. If you are now starting with Scala, you have more choices.

SBT ended up winning due to build plugins and cross compilation between multiple Scala versions.