at constant volume why the change of enthalpy doesn't equal the change of heat
At constant volume, the pressure is not necessarily constant. The pressure will usually increase when the system is heated. This makes the $PV$ term not constant. For this reason, you get:

$$\Delta H = \Delta (E + PV) $$
$$\Delta H = \Delta E + \Delta (PV) $$
$$\Delta H = \Delta E + V\Delta P $$

Now heat is just $\Delta E$, because there is no work. So,

$$\Delta H = q+ V \Delta P $$