Pure personal opinion of course: magicians actually have other things to do: spellcasting. Deckers are deckers. Take away the matrix and they are riggers without the drones or street samurai without being good at combat.
At the same time, the Matrix actually can affect the real world: you can hack turrets etc. The Astral Plane on the other hand, while being parallel and allowing the person on the astral journey to percieve the real world in a way, doesn't really allow them to affect it. It would take a lot more to tie it into gameplay. Though it would be nice if they did eventually with a DLC.
Or if one was to take it by pure maths: Third Edition Rulebook: The Matrix pages 199-230, The Astral Plane pages 171-177 (magic is 158-198). So the rulebook dedicated 5 times as much space to the matrix as to the astral plane, while the astral plane is only 6 out of 40 pages in the magic chapter.