The original SparkSQL was pretty much modelled after the Hive flavour of SQL, down to the available udfs. The compatibility was never complete and has somewhat diverged again with respective releases of the frameworks, but for the most part, Hive was the big data framework to beat at the time (2015-2016), and not everyone wanted to write Scala.

I think that now, maintaining that compatibility is less of a need for Spark and Hive has introduced a lot of goodies in the meantime, so there might not be a need for the SQL flavors to be in lockstep anymore.