The main reason Kepler can only see specific kinds of planets is that you need to see three transits before you confirm it, and that the further out a planet is

The main reason Kepler can only see specific kinds of planets is that you need to see three transits before you confirm it, and that the further out a planet is from it's star, the less likely it is that it will ever transit. These two combine to make planets closer to the sun much more likely to be seen. If there was another Sol in a perfect place and at perfect alignment and Kepler had been looking straight at it, by now we would have detected Mercury, Earth, Venus and Mars but nothing else.