You ask an important question. Some philosophers (realists) argue that simpler theories are better confirmed by the data and therefore more likely to be true. Other philosophers (anti-realists) argue that simpler theories are psychologically easier to work with and therefore more convenient for us, but not likelier to be true.

It is difficult to state exactly what counts as a "simple" scientific theory. Does it mean fewer causes and/or entities? Or something about the mathematical expression of theory? In any case, simpler theories are only preferred when all else is equal, and that is rarely the case. (We would love a simple theory of the causation of schizophrenia, but simple theories of the etiology of the disease have already been discredited.)

Some feminist epistemologists have argued that simple scientific theories are not inherently preferable to more complex ones. Helen Longino, notably, argues that simple scientific theories often reflect/express/derive from a hierachical ideology of single powerful causes, and that more complex theories (often reflecting a more egalitarian ideology of interactive causes) should be proposed and taken seriously more often.