How did Large sell out so quickly?

I saw it first thing this morning and decided I MUST have it NOW, which doesn't happen that often. And then I decided to see if my brother wanted any shirts so I could order them all together. I told him to leave me a note if he saw any he fancied. I didn't get off work until 11 and I went to Threadless first thing and now it's gone!

Eff.

And then I remembered that I had put it in my shopping cart just in case it happened to sell out, which I didn't really think it would that quickly. It was still there when I logged back in, but they still wouldn't let me order it since it sold out while I was at work. So much for my brilliant idea.

I have two options: buy a medium and hope I'll fit into it one day
or buy a hoodie. I HAVE been meaning to find an incentive to exercise and that hoodie is so pricey.