A bit is the manufacturers, a bit is the government, tires, axel strength, cooling, there really is a lot that gets considered. 

Those plus the chassis configuration 

What determines how much a truck can tow? Engine size? Transmission? Braking ability? 

Well all of those and more also the strength of the chassis the suspension system and the thickness of steel used

Here's more detail from Kelley's Blue Book on the issue (agrees withe above, but adds detail):

"How Do I Know What I Can Tow?

Unfortunately, it’s not easy to find the towing capacity of some vehicles. The amount of weight a vehicle can pull can be affected by its engine, transmission, and suspension. Most vehicles powerful enough to be used for towing heavy loads get offered with a selection of each of those components. Truck manufacturers also offer towing packages that can change axle ratios, tire types and include factory-installed hitches."

Braked vs. Unbraked Towing Capacity

Some trailers come equipped with their own brakes, connected electrically to the tow vehicle so that stepping on the vehicle’s brake pedal activates the tow vehicle’s brakes and the trailer’s brakes at the same time. A vehicle can tow a much heavier load under control if the trailer operates with its own brakes.

Thus, braked towing capacity, or the amount your tow vehicle can tow if the trailer has its own brakes, is higher than unbraked towing capacity, or the amount your tow vehicle can tow if the trailer does not have brakes. "

Most truck and SUV manufacturers maintain websites that can provide that information based on your vehicle’s VIN, such as Ram’s Towing Calculator or the Toyota Vehicle Information Lookup tool."

https://www.kbb.com/car-advice/towing-capacity-guide/