Because information asymmetry increases the cost of equity greater than the cost debt. One theory behind this is due to better creditor rights/protections afforded to debtholders relative to shareholders. Also, you can look at this from a pecking order perspective and an NPV one (e.g. if an insider has information about the expected cash flows of a project he will want to discount that at the lowest rate possible to achieve the highest NPV, which is first internally generated funds and then debt).