Because tax authorities don't accept unrealised gains as remittence unfortunately, so hodling is not an option for masternodes. By running one you are guaranteed a liability of around $2000 a year in my area (which is not one of the higher income tax zones) whether you cash out or not because the revenue is unearned, so it's 99% taxable. It is not compensation for the costs of supporting the Dash network (whereas mining revenue is and so is not similarly exposed to statutory selling pressure). The risk of capital loss comes on TOP of all this.