Because back then, flights were more expensive and it better suited the rustic charm ambience of the TV series to have him at teh port area I suspect.
Sealink - wow that does date you, they stopped in 1984 ! These days it is a large smart Condor cats from Poole that take 4.5 hours.
A very decent airport it is too, with regular flights in BA & FlyBe jets, as well as the private jets of the monied men.
It takes one of my Jersey friends less time to commute door to door to his Investment Bank in London via Jersey airport to City Airport on a Monday morning than it takes me to do the car+train+walk journey to my office just across the Thames from him.