The HSC results are released by the Board of Studies. The ATAR is released by UAC.
If both results were released at the same time, students may over-concentrate on the single value ATAR, without properly considering their achievement on a course-by-course basis. The earlier release of the HSC results allows students to digest and reflect on those marks, then process their ATAR on the next day.
Many students (the majority) are not interested in university, so the ATAR is meaningless for them.