Welcome to the community. To come to your question:
-Alternate airport: If the destination airport is closed, e.g. bad weather, one needs fuel to reach an alternate airport. That's a minor factor if one flies over Europe. But if you want to fly to Iceland and the airport closes shortly before your arrival, you may have to fly back to Scotland. Then many other requirements already spoken about.
-Engines need less maintainance if they can be used below maximum power = derated. An airline can choose a plane with different maximum takeoff weights (=MTOW). Lower MTOW is cheaper. It also lessens the landing fees. However the plane and the engines are the same. But the engines will not run at maximum power. It's called a paper derate.
If the airline some years later needs more power the airline for a (high) fee can choose the higher MTOW on their old plane.
A plane which starts below it's paper MTOW also doesn't use max engine power (of it's paper MTOW) to reduce maintainance. It can climb to high cruise altitudes faster, which saves fuel.
-I think the main factor however is cargo. In the US cargo has it's own planes flown at night, e.g. FedEx. Within Europe cargo is mostly carried by truck. US-Asia cargo often is carried by own planes with a stop in Anchorage. However if cargo is carried below 10 hour or so flights, airlines try to carry as much as possible in the belly of passenger planes. Europe-US and Europe-Arabian Gulf are just two examples.
The range Wikipedia/ aircraft manufacturer give you is with passengers and luggage only. One has to deduct reserves, wind, ...
But beyond this airlines like to carry cargo instead of fuel, e.g. A330s or B777s on six hour flights. Passengers, luggage and cargo are called payload. I'm not sure if toilet water, food and drinks and such stuff is also included in payload. The more payload, the less fuel and therefore range.
Check this page:https://bigsynthesis.com/understandinga ... ge-diagram
I read Air Canada was using B777-300ER on routes on which they could rarely sell all seats. No problem, they had enough cargo to carry. If they can sell the seat, so much the better. I think these flights were above twelve hours, but I don't remember exactly.
-Then there is availability of planes. Roughly speaking the longest distances flown will see the youngest generation planes. Whatever plane was used earlier on these routes will be used for shorter routes. E.g. an airline may have leased a B777-300ER for 12 years for a long range route. After the 12 years the airline replaces the B777-300ER with A350. What to do with the 12 year old B777-300ER? Similary A330s are replaced with B787s. So planes may be used on routes which they were originally not designed for, if they are cheap enough on the second hand market.
To get a rough idea for which distance planes are used, see the freighter's version range. That however may only be true where passengers and cargo is carried.