1. They might want to wait for currently unavailable options.
2. They might have reserved with the theory that they'd flip the cars and make profit.
3. A lot of people are willing to put down a refundable $1k, but when it's time to make an actual commitment, that's a bigger deal. I heard a rumor (again… rumor, not stating this as fact) that the Model X reservation -> order/purchase conversation rate was as low as 9%. Not 19%, or 49%. 9%. I've personally figured there's no way that more than 50% of the reservations would be orders *ever*, and I'd be kinda surprised if more than 15% become "First Production" orders.
4. A lot of people want to lease, and can't.