Because, in the latter case, you have to declare a function argument for /every possible option/ that you want your graphics API to expose, and you need to do this every time you add a new option.

On the other hand, declaring the options through composition means that the API for "plot" remains static, and adding/removing options can be done trivially without an API change.

Composition (rather than parameters) is also more flexible. Let's say you want to divide your plot into three sub-plots, two of which are 200x200, and another which is 200x400. How do you express this as a keyword parameter? In composition, you could do something like:

plot( ggsubplot(ggvsplit(ggsize(200,400), gghsplit(ggsize(200,200), ggsize(200,200)))) )