If I'm not mistaken news about the update are not tied with the update itself — someone from Allegorithimc should add them first. There could be no news on Steam about the update for some time, while the software itself will be already updated.
In any case, that's weird. You could try to delete your SD installation and try to download it again. If it will not download 5.6, then definetly something is wrong.
Alternatively, you could link your Steam account with your Allegorithmic account, so you would get a stand-alone license key for SD. You would be able to install SD 5.6 from the website, and use it before the issue with Steam update will be figured out (though I understand if you want to keep everything in Steam, but that's just an option for now).