Rarity of the wood (could be due to harvest restrictions, import restrictions, not many of the trees, not many suitably-sized pieces), rarity of the grain pattern (more highly figured or patterned grains are unusual, and some species tend to have more knots/burls/figured pieces than others), and difficulty of the wood to work (some woods are more likely to split or chip; curly grained pieces tend to be more difficult to work with than straight-grained pieces of the same type).
So supply, prettiness, and workability.