Launch vehicle engine design is a complex thing, with many trade-offs. Propellant selection, the type of propulsion system, and size are just some.

A larger engine needs fewer engines per vehicle, and all else being equal, saves weight and sometimes cost per vehicle.

However, more engines increases redundancy. Developing and testing smaller engines saves a lot of development money over very large engines. More complex high-pressure solutions are more efficient, but increase development and manufacturing costs.

SpaceX's engines have a robust design with a "pintle" injector and very little exotic tech.