Screenwriting is in many ways a matching game-a dating service for ideas, thoughts, and dialogue. Screenwriters must be able to match genres and ideas to be successful. A groundbreaking story that's written in the wrong genre won't see the light of day; this is a shame given the idea's core quality!
Take the well-made, high-quality, and financially successful thriller/mystery/hybrid Shutter Island from Martin Scorsese and Leonardo DiCaprio. The film kept most viewers on the edge of their seats, was entertaining, and aged impressively. Similarly, its core idea was second-to-none. However, if the work were shot as a comedy, with DiCaprio being halted by clowns, balloons, and zany characters every five minutes, it would have been plainly horrific. While this outcome is far-fetched, it must be remembered that it's far-fetched because such scripts never come close to being made into films!
Thus, it's imperative that script ideas be built upon and written with a suitable genre in mind. Only then will a script reach its full potential, and similarly, only then will aspiring writers have the best chance of seeing their work commissioned and eventually made into a full-scale film on the big screen.
But more important than the following effect of matching ideas and genres regarding success is the subsequent effect of doing so concerning quality. By failing to pair ideas and genres correctly, your quality will suffer. Furthermore, you will be doing yourself a disservice as a screenwriter and will dramatically decrease the likelihood of selling your screenplay.