As @lihaoyi said, the ScalaFiddle experience is really neat, and currently significantly better than Scastie for language demonstration purposes. Unless Scastie catches up in the meantime, I think it would actually be detrimental for the promotion of Scala to use it on the frontpage instead of ScalaFiddle.
Many languages have very slick online demonstrations (such as BuckleScript), so it seems important not to show off a more sluggish experience (in comparison) –– even if it means trading off things that matter less.