They represent love. Love requires contraction. The change of name indicates contraction. Isaac is greatness. Greatness is contraction.
Contraction needs no contraction into a new name.
Jacob was a mixture of love and greatness. From the side of love he needed to change his name in order to contract and receive the love. From the side of greatness which is pre-contracted, he needn't change his name. When Israel are living up to themselves, they are called Israel, a reflection of G-d's love. One who fears, sees.