To be more precise, they already had the port, but were afraid to lose it to the new government of Ukraine.
This is also a big reason why Crimea, by and large, supported annexation - it has very large proportions of the population that are either active or retired Soviet/Russian navy.
The Russia Empire and the Soviet Union ran concerted efforts to Russianise the population of Crimea by various means, like moving in ethnic Russians, removing the locals (to put it mildly) etc.
removing the locals (to put it mildly) etc.
Virtually the entire indigenous and otherwise non-Slavic population (some 30 percent of the total population of the peninsula) to be precise, according to this graph:
https://upload.wikimedia.org/wikipedia/commons/6/6a/Ethnic_P...
The original Russian colonization of Crimea brought in many Ukrainians as well, though, so that alone did not make it lean Russian so strongly by itself. The naval facilities in Sevastopol, though, meant a lot of military personnel with their families from all over the country would come and settle in the city, and that specifically tilts the popular opinion there today strongly towards imperial Russian irredentism.
