There is no trailer for Knytt Stories. The reason is quite simple to explain. Knytt Stories is a free game. A trailer is meant to be used as advertisement, which is useful when you want to actually sell a game.
Although a demoversion of a game is also used as marketing tool, the sole purpose of a demo version is to ensure the game runs on your computer. Installing a demoversion is often a step too far, so people have to be made interested otherwise. Trailers are easily shared and watched which is why they exist. But again, they're a marketing tool, and for a marketing tool to be useful, you often have to be selling something. This is not the case for Knytt Stories. The game is free which is often enough to tell people to try it anyways.