I can second just about every explanation that has been provided as one I heard.

I've also been told they don't stock as much for early flights.

The early flight thing and margaritas tends to be one that they don't always have lots of mixer for.

It really is easy to see how hard it would be to stock up for all possibilities. When I've asked, "how many do you stock?"
I've only every been told "more than anything else, but it isn't enough. But usually I'm told they're not allowed to say.