There are many factors involved in the desicion making of which aircraft to assing to which route. Firstly, they must decide on an aircraft type. Obviously, the easiest is to look at the number of passengers; the more passengers, obviously the larger the aircraft that is needed. Reservations will either sell tickets limited by the seats available on a certain aircraft type, but sometimes when it is easy for the airline to put on a larger aircraft at short notice, the airline can sell more tickets and simply change the aircraft type needed.
The first restriction is that the bilateral agreements may only allow an aircraft of a certain size, so even if you have loads of passengers wanting to fly, if the best you are allowed to do is carry 200 people on any one flight, there is no point in putting on a 747.
Secondly, freight is a big earner for many airlines, so when we recently got the 777 from Kenya Airways in HKG
rather than the 767, it was not for additional passengers, but an extra large amount of freight.
For a short haul flight, at Cathay we sometimes have a route operated by 747-400 on one day, A330-300 on another, 777-200 on another, 777-300 on another and A340-300 or even the A340-600 on the other. Many of these flights depart and will arrive back in Hong Kong either the same day or the following day with the same set of crew. A problem arises when an aircraft needs a crew change. No use sending a CX
A340-300 to FRA
instead of the 747-400 on one of the days because there won't be an Airbus crew to bring the aircraft back from FRA
...and the 747-400 crew due to fly back to HKG
that day won't have a plane to fly! This can apply to cabin as well as cockpit crew. If an aircraft swap is needed, it needs to be arranged well in advance.
Sometimes the engineers may not be qualified for another type. For example, if you had previously sent a CX
A330 to Colombo, sri lanka, the local engineers would not have been able to fix it if anything went wrong because they are only 777 qualified. This pervents the company from sending anything but the 777s on this route. (The engineers there now have Airbus licences too).
As for individual aircraft, a certain twin jet for example may have had it's ETOPS tempoarily downgraded because of equipment failure, e.g. APU failure, which would limit that aircraft to non-ETOPS routes, so that particular plane cannot fly the 'normal' route it would have done. Other aircraft may have a different cabin configuration which the company may want to send on a certain route. Many companies send their long haul aircraft on short haul flights during the day, however if you have a new cabin, you may want to showcase it and keep it on as many long haul flights as possible instead of the short flights.
Some airlines like CX
have their aircraft sitting around for a significant part of the day. CX
uses this opportunity for the aircraft to have maintenance done on it, and therefore an aircraft with lots of little things to fix may be sent there in preference to others. Maybe it is cheaper to wash an aircraft at city Y rather than City Z, so a dirty aircraft is deliverately sent there for a wash.
There are many many things which decide why an aircraft type or individual aircraft flies to a certain destination. I have touched on a few of the reasons here....there are many more, but hope you get the idea!