The Russian fleet was using Ukrainian ports in Crimea pre-2014 by agreement with Ukraine. The Sebastopol bases were Russian military enclaves inside Ukrainian territory.
Why was this not a sufficient arrangement? It had been the status quo for over 20 years.
If the population of Crimea had wanted Crimea to return to Russian control, they could have peacefully lobbied for that, and for a referendum to prove their case during that 20 years. As far as I know there was never such an initiative in all that time.
Indeed, Russia could have openly supported such a civic initiative and lobbied for it internationally, and/or offered to negotiate a change of arrangements with Ukraine, such as an outright purchase, subject to a referendum. There was nothing like that.
Sorry, but Russia was not for sale as was Ukraine. Most of what Ukraine sold off to the West is now or soon going to be under Russian Management. Ukraine will be the worst investment the western nations ever made - but anything left of Ukraine after the war will be the 51st State of America because they are bankrupted and owe more than what will be left.