Only TV work is allowed to be done. Now, let's see the result of constant temperature and pressure. In an isolated system, the number of moles stays the same. And that's because there is no exchange in mass. So, if the number of moles stay the same, if the temperature is constant and pressure is constant, and according to the ideal gas law, the volume remains constant, so there is no volume change. So, according to the formula for change in enthalpy, if there's no volume change, then this guy, the PV work done, is zero.