Why do you think Rome and Venetia initially remained separate after unification?

You may be referring to Venetia as the Republic of Venice. Venice fell when forces under Napoleon occupied the city in 1796. The French and Austria agreed for peace in the Treaty of Campo Formio, in which both nations agreed to share the territory of the ancient city. So Venice was ruled by the French and the Austrians after it fell, and not by Rome. Hence, the reason why Venice was not considered a part of Italy at that time.