Hello,
It's scarcity and the law of supply and demand. How many two-letter possible combinations are out there? Not many, right? How many companies have you seen using a two letter domain and what size are those companies? Unfortunately a two character domain in .com is out of reach for most of businesses, a three letter? need some serious cash... four letter? it depends on its brandability but a good one usually goes in five figures.
The market is scarce, that's why they are expensive...