How do airlines choose which planes to travel with?

Like from Toronto to Cuba I took Sunwing boeing 737, but when I went again using Air Canada they used an Airbus A319.

How do they decide? Like why wouldn't they use a boeing 747?

Depends on contracts and which model plane the airline likes and purchases. It's like a company buying company cars, they'll shop around and choose which make/models they prefer for their needs/budget.

Executives ultimately decide which planes to order and buy based on what they believe their needs are in the coming years and the costs (upfront and long term) of each type. Which plane makes which route is based on overall routes and how they believe they can best maximize their revenue.

There are several factors, including:
How many people they think will fly on the flight
Cost of the plane (to include what discounts they can get out of the manufacturer)
Operating costs of the plane (fuel, number of flight attendants required, etc)
Brand loyalty (which can factor into the cost of the plane)
Commonality (for example, an airline might fly all 737s even it it might be too big for some routes or too small for others simply for the efficiencies in crew training and maintenance)

Airlines have a fleet of planes to pick from.
Every Airline does not have exactly the same fleet of planes.

They have many routes to allocate their fleet. The number of expected passengers, the distance of route are first determined before selecting an available plane for the route.
They can change the type of plane as other fleet requirements change over time.

Like why wouldn't they use a boeing 747? WHY should they?

There are currently no Boeing 747 commercial flights offered by US or Canadian Airlines.

Air Canada became the first Canadian airline to introduce 747 service across Canada and to points in Europe during the Spring of 1971.

Planes with lower operating costs have been developed since then. Air Canada stopped using 747's about 20 years ago.

Sunwing Airlines is Canada's youngest and most fuel efficient fleet - comprising over 40 Boeing 737-800 aircraft They do nott have any 747's in their fleet and have no need for one at this time.

They tend to call flying program their operation.
Airlines choose their fleet based on where they are operating to and the demand for each route and capacity and capability of the aircraft. In other words some purely domestic carriers, will have similar size aircraft, wth a mix of seat capacity, so when demand is higher for one destination, they can choose a larger aircraft, and in a less demand period, use a smaller aircraft, even across the B737 or A319/320/321 fleets there are varying capacities. The distance aircraft can operate is another factor, hence airlines who operate domestically and internationally have smaller Airbus A320 or Boeing 737, as well as larger B747, B777 or A330 aircraft for the long haul destinations

The traditional airlines such as Air Canada, United, American and Delta have a number of different sized aircraft. They will use the aircraft that is optimal for each route they fly. This means, the aircraft that is big enough to carry the passengers who want to fly to that destination but is not too big that the plane is flying half-empty. They also want to use the most fuel efficient aircraft they have for that route as well. Know that airlines do change the type of aircraft they use throughout the year. So during the busy summer months, the airlines may use a larger aircraft on a particular route but during times where fewer people are flying, they downsize to a regional jet.

As for why they don't use a 747, it is because there's not enough demand to justify using that plane or the route is more competitive or is flown more frequently that the airline chooses to operate 2 smaller aircraft instead of offering 1 flight a day using a bigger aircraft.

As for what type of aircraft be it Boeing, Airbus, Bombardier or Embraer, airlines will negotiate with the aircraft manufacturers. Things such as price and maybe service/maintenance packages agreements are talked about and after the airlines look through the proposals offered by the manufacturer, they then decide which aircraft manufacturer to go with.

Flight planning is the process of producing a flight plan to describe a proposed aircraft flight. It involves two safety-critical aspects: fuel calculation, to ensure that the aircraft can safely reach the destination, and compliance with air traffic control requirements, to minimize the risk of midair collision. In addition, flight planners normally wish to minimize flight cost through the appropriate choice of route, height, and speed, and by loading the minimum necessary fuel on board. Air Traffic Services (ATS) use the completed flight plan for separation of aircraft in air traffic management services, including tracking and finding lost aircraft, during search and rescue (SAR) missions.

The ones which make the most profit on a route.

It all depends on their 'Purchasing Budget' and who can give them
the best deal for the number of planes they want to buy.

Based on supply and demand