I just meant that it doesn't take more muscles to frown.
Anyway currently no one knows "why". Scientists can't even decide what they are measuring. Are eye muscles included in a smile? Or is it just the corners of your mouth that need to lift. When you frown does your forehead have to go all wrinkly?
The answer to your question is usage. Usage is how words are used in a given context. There are no mandates in English grammar that specify on and in must be used as they are in these instances. Why we say "in a movie" and "on TV" is merely the way the language has developed over time. Both are grammatically correct, but the prepositions in these phrases are not exclusively correct. Any arguments for why on and in are not interchangable in these instances will most likely employ rationalizations about how the phrases relate to context which is the realm of usage, not strict grammar. Context is important as it gives us a frame of reference for meaning, but the strict rules of English grammar may allow for the same thing to be said in many different ways, but the context of usage may require that certain meanings be phrased in a specific way so that the meaning is clear.
Buy a bible then you don't have to go to the library.
