This is because in Chinese geomancy great importance is placed on the yin-yang philosophy. According to this philosophy bars which are solid are yang and thus odd; while spaces are yin and even.
因為按照中國傳統有關風水的說法：陽屬奇數，陰屬偶數，而直條是實的，屬陽；洞是虛的，屬陰。