Probably because alcohol requires an above-21 person to sign and receive the delivery, which adds complications to GSX if no one is home when they deliver.
They do now. I've ordered it. If I had to guess, I imagine it's not a legal requirement you want to tackle right away when you're just getting started.
So they do! That's interesting. Did you have to be home to take delivery for it?
Also it doesn't look like they've added it for Costco which is a shame.