Why is it so hard to buy a Surface Pro 2?

I'd really really like to buy a Surface Pro 2 (with 256 GB storage and 8 GB RAM), but unfortunately it's been unavailable from the Microsoft Store for weeks now. I check the store every day, but it remains "out of stock" all the time. (At the moment not a single of the four configurations is available)

Can anybody explain, why Microsoft can't make them available again? If people want to buy, but they won't sell, it's no wonder that the sales figures for their tables are abysmal.

By the way: I'm from Austria, so I naturally check the Austrian Microsoft Store website. But as far as I can tell the 256 GB Surface Pro 2 hasn't been available in other countries either for a long time.