Because they couldn't have them in Apple stores until THIS week. Clearly nobody has gotten a higher-spec'd MBP yet from their order. So they hadn't ramped production to have them available for stores last week. They had to put their cards into one model per size to be able to say "available today"... if they'd had the ability to have all models produced they would have. But this way they said "Available today" so you can order it thinking they're making it when in fact the nature of product manufacturing is it takes a week to ramp up manufacturing and distribute a product like this. They're not going to give up on a week's worth of sales this time of year.
This is fun. So let's continue down this path. All four of the stock products (2 11.6 models, and 2 13.3 models) show 'shipping within 24 hours. As soon as you change one thing (RAM, processor, drive space), the shipping moves to 1 - 3 business days. So, if they have them all already made, why hold out? What's the marketing plot on this one?