Airlines serving islands and coastlines tend to operate similar aircraft across very different parts of the world. The convergence comes from a set of environmental and economic pressures that behave the same everywhere.
Salt attacks the airframe continuously
Aircraft based in marine environments live in air carrying salt, which accelerates corrosion on structures, fasteners and electrical connections.
Maintenance programs respond with more frequent inspections in known problem areas, along with washing regimes and protective treatments applied on a schedule.
The consequence is a higher maintenance cost per flying hour than an identical aircraft based inland would incur, and that cost has to be recovered in fares.
Runways set the upper limit on size
Short island strips restrict which types can operate, and performance is calculated for the actual runway rather than for the aircraft's best case.
Wet surfaces, tailwinds and high temperatures all reduce the margin further, so an operator sizes the fleet for the worst regular condition, not the average.
The result is a preference for aircraft with strong short-field performance even where that means giving up range or seat count.
Thin markets favor frequency over capacity
Island populations are small and demand is seasonal, so a large aircraft once a day serves the market worse than a small one several times.
Frequency also matters for freight, mail and medical transport, which islands depend on and which cannot wait for a weekly service.
Smaller aircraft flown more often therefore fit the demand pattern even though the cost per seat is higher.
Support has to be self-contained
A fleet of one or two types simplifies spares, tooling and engineer training, which matters enormously when the nearest alternative supplier is across water.
Operators hold stock locally because a part flown in can take days, and an aircraft waiting is a large share of a small fleet rather than a rounding error in a big one.
Engineers are often based on the island for the same reason, and their qualifications are tied to the types operated, which makes changing aircraft a training program rather than a purchase.
Weather narrows the operating day
Coastal fields deal with crosswinds, sudden showers and sea fog, and aircraft have crosswind limits that are absolute rather than advisory.
Afternoon convection in tropical regions closes fields regularly, which is why island schedules concentrate in the morning and thin out later in the day.
Travelers connecting onward from an island are working around that pattern whether they realize it or not.